My 2008 Starcraft 14RT uses OSB on the floor with some kind of black painting on the bottom... Have had great experience with this in my many off-road trips...
The Starcraft roof is either Fiberglass or ABS membrane and the area below it is filled 2-inch foam. Then I have regular 1/4-inch ceiling made of wood paneling.
Seems to hold up great.
I also have a 13,500 Roof mounted air conditioner on top and is supported with two cross ceiling metal sections... Looks like this but I also have a second cross member section on the other side of the air conditioner...