I wonder why rv manufacturers put batteries inside in a vented box. I've had three TTs with batteries inside. One was in a closet in the bedroom, the other two were located under the kitchen sink in the cabinet. All three had a plastic box with a hose attached to the side with a small round vent cover. I know the rv industry is highly unregulated but if it were so dangerous would they be allowed to get away with it this long?
That said, I too want to do the same thing. My TT is tongue heavy. I want two additional batteries and move all of them to the rear using a reinforced bumper and a long battery box. Also, to change the LP tanks to the underside like on a Class B or C and then use the hitch for an encased generator.
They don't make them the way I want, so we'll just make it the way I want. Anything is possible and in some cases it has a cost associated with it. We are on the road a lot and we are boondockers.
I think the OP has a great idea and I hope he continues with his project. It may require some consultation from experts including a call or emails to the manufacturer.