<<<I'm a little concerned about the added weight
of the treated plywood, but didn't see any other option. >>>
I would be more concerned about the TREATED plywood, as it is toxic, and shouldn't be used anywhere in the interior.
As long as you seal it, exceptionally well, inside everywhere, maybe you won't have any repercussions.
My FGRV has fiberglass underneath, as it is all part of the hull, to seal it from the elements. I know that some FGRV owners have fiberglassed underneath theirs, when replacing the plywood floor, to protect it.
Congratulations on your Roly-Poly. It is a lovely trailer, I would like to have one of those, but my 4 cyl. car can't tow anything bigger than 13'.