I wouldn’t have a problem using PT wood for this repair since it’s not in the living area, although I would try to use some untreated white wood between it and the aluminum framing, and use the proper fasteners as well. I’m not sure PT wood is actually needed though, because I’m quite certain that I could rebuild the bottom so that the wood frame would never get wet.
Glassing it in isn’t necessary either. Covering the exterior luan plywood with FRP would be a good thing though. In fact, I’ve done that to the undersides of the “wings” and the sides of the “tub” on my camper. Someday I’m going to cover the bottom of the tub with it as well.
I wouldn’t recommend using the Trex type plastic wood either. It isn't thermally stable enough to use for RV framing IMO. It shrinks and grows too much.
Just my thoughts after doing some extensive repairs on my wood framed TC.
:):)
