I don't remember reading about this "option" while I was going through the TM for my M101A2. Since these trailers are commonly known as "trailer salad", it's hard to know exactly what was done. I'd have to check the TM again to see if there was something there about a built in fuel tank, but I don't recall it off hand. I'm sure someone with more intimate knowledge of these trailers will chime in at some point.
It could be a generator trailer (M116) that has been modified for a M101 style box and fueling capacity. That is just a guess though, I don't know the real reason to have a separate fuel tank on a trailer unless there was something mounted in the bed that could draw from said tank.
Oh, and welcome to the OD Disease, it always starts small...