The good news for you is it sounds like this truck is not being sold because the owner knows a head gasket or transmission is on the horizon. This truck has possibly been maintained and used as if it was a forever truck. That said, as CPT has stated, this is not a 2021 toyota tundra towing a midsize airstream.
For a travel trailer, this is likely too much truck unless:
1) The TT is 35' or more, 10k # or more.
2) You plan to do long trips, where the comfort of having too much truck, longer wheelbase, and fuel range are worth it.
3) You realize the extra expense going into it, and can rationalize the $$.
4) You aren't planning to daily drive it in urban conditions, or otherwise realize that a DRW CCLB truck can be cumbersome.
I've sunk excessive money into two trucks now in the name of RV pulling, so I get it. Breaking down with my camper and family far from home when trying to get back home for work is less than ideal, and so I maintain proactively and carry spare parts.
Google "diesel mechanic" in the area where the truck is being sold and see if you can get a pre purchase inspection. If you are unfamiliar with the platform it will be well worth the $250. Try to find an independent that will allow you to look over the truck with them.
The FASS lift pumps are overhyped and overmarketed, but they are good. I wouldn't pay a shop to do one on a duramax. You can do it yourself and do it cleaner. Airdog and Kennedy also sell kits, you can DIY one together that fits your desires. FWIW, the FASS was the reason our truck and camper burned to the ground lass summer after our accident. Make sure the fuel pump is on an impact switch.
The Bosch CP4 injection pumps fail catastrophically and without warning. When they go, metal goes throughout the fuel system. Injectors need to be replaced. All fuel lines get pulled and cleaned. Tank gets pulled and cleaned. Five figure bill if you're paying someone to do it. It was the weakness of the LML and is now becoming a sore spot for the Ford 6.7. Ram used it for a few years, issued a recall, and converted trucks to the predecessor, the Bosch CP3. The LML can be converted to CP3. CP3 pumps wear out, but they are more resilient to bad fuel. When they go, it's gradual (typically you start seeing a low rail pressure code under heavy use) and it doesn't wipe out the entire system. Ford still uses the CP4 but there is an aftermarket conversion to a Standyne pump that is gaining popularity. GM is now using a Denso system on the L5p.
The Allison internal filter isn't a big deal. Do the spin on filter ($15) and clean the magnet every other oil change. Drain the pan and refill with fresh ATF every fourth oil change. It is common for the allison to leak a little ATF from the wiring harness connector seal that goes through the case or the shift cable. They make a kit to rehab the connector seal if it really bothers you. I think the valve body has to come down to do it. Full rebuilds/upgrades can be done on the Allison for far less than a ford 6r140 or ram 68 rfe, so that's nice, but is unlikely to be needed unless the truck as been tuned and beat on.
As was mentioned, the Allison T87 TCMs do eventually fail. The electronics inside the box go bad due to heat and vibration. It seemed to be more of a problem on the LML trucks than the prior generations. It was especially painful post covid (chip shortages) when they could not be sourced. That has been somewhat resolved. The TCM is right behind the radiator, it's an easy roadside swap if you choose to carry a spare.
The transfer cases were solid by this generation. No need to worry about pump rub or excessive snap ring end play like on the earlier duramax NV261/263 cases. Do fluid when you do differentials. Any good synthetic atf is fine (ATF+4, Dex VI, etc.) Syn 5w-30 if you want to be unique. 2 quarts. Easy peasy.
Fuel filter (under hood) on a reasonable interval but carry a spare and tools to do it (WIF sensor wrench and strap wrench) on the road in case you get a tank of bad fuel far from home. If you add additional filters ahead of this one, carry spares for those instead.
The front differential has a drain plug and a fill plug. You should change regularly even if you never use 4x4, because they don't get hot enough to drive out moisture. The rear 11.5" AAM axle housing on the GM has a drain plug on the bottom of the casting. It is still nice to open it and examine for debris on a new truck. GM uses a cork/flat gasket, but you can source a mopar/ram 11.5 AAM gasket that is reusable. Easy peasy. Keep an eye out to make sure the rear axle backing plates remain dry, else you have an axle seal repair on your horizon. They are usually slow leaks before they are big leaks.
The earlier duramax trucks did not have coolers on the power steering systems. They may have added a cooler by 2012. This meant the systems ran hot causing the hose crimps to leak. Hydraulic hoses are commonly leaky but are easy to source and replace. The extra heat also meant the hydroboost didn't last forever and if the owner let the fluid deteriorate that PS pump would meet the same fate. And then there's mechanical wear on the steering boxes.
If water pump has been changed, that's a bonus. They don't go often (like on a cummins 6.7) but when they do, it's a larger job.
The rest is typical GM truck stuff, mostly within the realm of DIY. Suspension bits. Interior rattles. Cab mounts. Rust. Gauge backlight bulbs.
If it were me, and everything else about the truck was excellent for my purposes, I would gradually tackle the fluid, filters, and any cosmetic bits. I would get a lift pump and extra filtration on it ASAP, along with an S&B fuel tank (unless prior owner already has slip tank in the bed.) From there, I would plan on spending my free time in the first year tightening up the front suspension/steering. I'd want to have a CP3 conversion done within the first year.
