Your rv converter is likely the Wfco.
These are famous for never going into their 'boost' stage, where they bring the battery upto 14.4ish volts at their maximum rated amperage.
Even if they are rated for 35 amps, if it only ever seeks to hold 13.6v, it will take 36 hours or more for the battery to reach full charge, if the battery were still healthy, which it would not be for long if cycled deeper and only the wfco used to recharge it. When not healthy, 13.6v can take significantly longer to get the battery to the 99% charged range and 100% is likely be impossible, no matter how long it is applied.
When you plug in your smart charger, and this wfco is running, it is likely the smart charger will see the slightly raised voltage from the wfco converter, and decide the battery is already fully charged, and then it will not apply its full amperage, and it is pointless to run a generator if the battery is not being held at absorption voltage, 14.4 to 14.8v.
It is far more damaging to a lead acid battery to begin another discharge cycle from a lower state of charge, than it is to hit it with as much amperage as it can take, voltage limited to 14.8ish@77.
A healthy flooded marine group 27, discharged to ~ 50%, will accept 20 amps for about 25 to 35 minutes before voltage rises to 14.4v, at this point less and less amperage is required to hold 14.4v, and about 3.5 hours later amps will have tapered to less than 1, and the battery will be nearly fully charged. if less than healthy or less than 14.4v is held it will take significantly longer.
When sulfated, which it will be after even one PSOC cycle it will accept 20 amps for far less time before voltage rises to 14.4v and then take significantly longer to approach 100%.
Any discharged battery is sulfated, though the term 'sulfated' generally means, to Joe Blow, hardened sulfation, which is difficult to return to solution. The longer the battery sits less than fully charged, and the lower the state of charge it sits dischrged, the harder the sulfation becomes, and the longer it takes to return it to solution, when held at charging voltages, an there is no guarantee it is possible to do so.
When camping with just a single group 27, and the wfco converter, and starting the generator to recharge, I'd disconnect the wfco and use the 20 amp 'smart' charger, as it should at least put out 20 amps until battery voltage reaches the mid 14's, at which point it will be 75 to 85% charged. The 35 amp wfco will only be faster for the first 2 minutes, before its 35 amps can get the battery to 13.6v. It depends on teh size and distance of cabling from wfco to battery how long it takes.
getting from 80% to 100% will take no less than 3.5 more hours, assuming the charger is holding the battery in the mid 14.v, which is extremely unlikely.
Running the generator just to reach 100% is a waste of gas, and silence, so the highest amperage charger possible to get the battery to the highest state of charge possible, before shutting down the generator is ultimately going to be best for the battery.
After 5 or 6 partial state of charge cycles, beginning the next discharge cycle before 100% is reached, has a battery punch drunk. It will not just be starting its discharge cycle from a lower state of charge, it will act like a much physically smaller battery, a more resistive battery, that is also less than fully charged.
It makes no sense to run a generator to charge the battery if the charging source is only going to seek and then hold sub 14.4 volts, unless you like the sound of your generator, and to irritate your potential neighbors, and to waste fuel.
A flooded group 27, even if it says marine/deep cycle, is hardly built like a true deep cycle battery. it if were then it would have such a low CCA rating as to be nearly useless if ever asked to start an engine other than when still relatively new and fully charged.
I've cycled dozens of marine flooded group 27s over the years. By and large they are very difficult to truly fully charge and this gets worse and worse as they age, but even from cycle one requires more than expected, more time at 14.4v plus.
They are hardly great batteries to cycle, but especially when only PSOC'd, partial state of charge cycled. The only way to get good lifespans from them is to insure a true full charge occurs when the camping trip is over, and the only way to do this is with a charger able to not only hold ~14.8v until amps basically stop tapering, which can be 7 or 8 hours worth, but by then applying a forced overcharge with voltages in the 15.5v to 16.2v range after that point. It can take anywhere from 15 minutes to 6 hours held at these 'equalization' voltages for specific gravity of all the cells to max out, and for the battery to be truly fully charged. Generaly the more PSOC cycles accumulated, the longer it takes, and EQ voltages should only ever ba applied after the battery has spent several hours held at 14.4 to 14.8v first.
When one uses a hydrometer and dips the cells on such a battery after a smart charger decides to flash its green light, one will find specific gravity well below the 1.275 range. A battery PSOC'd and then only 'smart' charged, is doomed to live for a fraction of the cycles it could accumulate if it were truly fully charged after a half dozen PSOC cycles.
But for many campers the battery might live for 2 years in their usage and this can be acceptable, a variable benchmark, opinion, philosophy.
When one gets home and can plug in and has all the time in the world to recharge, then one might say slowest possible charging is always best, but it is likely the PSOC'd battery, even after a week at 13.6v, will still have specific gravity in the 1.255 range or worse, where 1.275+ will be considered fully charged.
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Get the above hydrometer if you do not wish to remain ignorant as to state of charge, but be warned, you will get frustrated, as returning your marine group 27 back to 1.275 or higher on all cells, will be a lesson in futility after many PSOC cycles occured, and a so called smart charger is the only available charging source.
The WFCO converter is also known, not only for almost never seeking 14.4v, even with well depleted batteries, it is known for its high failure rate. An Iota DLS-XX converter is a good replacement, and the progressive Dynamics PD series is in my opinion better. The PD9245 has 45 amps available, and is still an automatic 3 stage charger, but with the 'pendant' one can press the button and force it to seek and hold 14.4 volts for 4 more hours, or force it to 13.6v mode if a quick recharge is not desired, or force it to 13.2v mode if one knows the battery to be close to full and wanting it to be more a maintainer.
The Iota DLS series will seek to bring the battery to 14.8v, and then hold it for 15 minutes, then drop to 14.2v for X amount of time.
Don't fear high amp charging sources on batteries deeply cycled, especially when camping several nights in a row.
Batteries accept as much as they want at the voltage reaching the battery terminals, upto the maximum amperage rating of the charging source.
The voltage should be regulated to never exceed 14.8v or so.
A charging source which can bring the battery to 14.4 to 14.8v in PSOC usage, as quickly as possible, will get the battery to as high a state of charge as possible while the generator runs, and keep PSOC cycling from shrinking the battery as quickly, while also providing more usable electricity that night to keep the furnace fan running or the inverter low voltage alarm from screaming at 3:30am.
When you get home then low and slow is fine, but insure that specific gravity is maxed out, or amps taper to low levels at 14.4 to 14.8v.
Amps tapering to low levels at 13.6v or below is not indicative of a true full charge. it could be, but likely, is not.
If you find your group 27 to be inadequate or needing frequent replacement, then 6v golf cart batteries(GC-2) are true deep cycle batteries, two are needed, wired in series for 12v, and will last significantly longer before you notice they are underperforming, and if treated right, they will last 3x as many deep cycles as a group 27 treated right. They are also likely 10 to 15$ cheaper than a group 27, but again 2 are needed and they are a few inches taller so might not fit your stock battery tray.
The group 27 flooded/wet marine battery is kind of a joke on the RV owner who camps without electrical hookups. Some rv dealers will save even more money and provide group 24's which have even less capacity.
I got so sick of so called smart chargers dropping to lesser voltages and sulfating my batteries prematurely, I now use a 40amp or 100 amp adjustable voltage power supply. I even hook either of these upto smaller 18 or 22 Ah AGMS, 'motorcycle sized' batteries. They accept what they want at the voltage I choose. Choosing a high voltage when they are well discharged can have them accept 5 or 6 times their 'maximum' recommended rate.
Choosing a high voltage when they are at high states of charge will never have them come close to this maximum rate for more than a second. Generally I will allow double the maximum rate when well discharged, unless I need to return them to service ASAP, then I crank voltage to 14.8v and let them drink like a sailor.
Use an ammeter, in addition to the voltmeter.
A battery accepting 5 amps at 14.4v is not fully charged, a battery accepting 0.5 amps at 14.4v is, or nearly is.
14.4v alone, tells the user little to nothing.
A 5 amp charger applied to a depleted, healthy group 27 is going to take many many hours to get the battery upto 14.4.
My group 27 TPPL Northstar AGM when new and healthy, depleted 60 of its 91 amp hours, would accept well over 100 amps for several minutes before voltage at the battery terminals, rose to 14.7v.
Expect a healthy well depleted flooded marine group 27 to accept 50 amps, easily. How much amperage it can accept from a high amperage charging source that is seeking high voltages, when well depleted, is an excellent way to gauge its health, its kind of like the carbon pile load test, in reverse.
