I would just use an LP generator designed for RVs and hook it to your existing propane supply. That would eliminate having to buy and install a gasoline tank.
Pick a storage compartment, partition it off. line it with sheet metal, and install the unit. The Onan unit in my TC has installation instructions with it. It would make a good guide for cutting the hole in the floor.
On a TT I would mount the gen. forward to be nearer the propane and battery even though this would increase tongue weight. Unless you plan on running it all night for AC while you sleep and your bed is in the front. In that case I would think long and hard about putting it at the opposite end of the trailer.