well I was wondering the same thing. couldn't find an answer but the FVP group 31 "dual purpose" sure does seem like a tank. "maintenance free" and has the little green SOC eye for what its worth. So I picked up 2 during their recent 11% sale. I use them to power a 12VDC/120VAC inverter that in turn runs my sump pump. The previous batteries were JC group 27 "marine deep cycle" (from costco) that were going on 9 years old. One of them finally sulfated, so time to replace the pair.
So far they seem to have some serious cranking capacity. My setup draws about 100A @ 12VDC for 16 sec while the pump runs. These 2 FVPs in parallel hold 12.15V during that discharge. The old pair of JCs would drop to 10.35V before rebounding to 12.8 after the pump stopped. Time will tell how long they last, but so far I like 'em.
I use the "marine deep cycle" or "dual purpose" units as opposed to "true" DCs because when the pump kicks on it's more like a starter than a low/long draw. But it could potentially need to cycle hundreds of times between charges, so it needs to have some deep cycling ability as well.
One thing I DONT like is no mfg date on the battery. I like to get them with the same manufacture date (or close to it) for parallel connections. So I did measure the terminal voltage of both batteries before buying to make sure they were close. 12.83 vs. 12.78. close enough.