Yes, there is plywood in the middle of the floor, and once it rots, you have a BIG problem. So keeping your trailer dry is critical. Read up all the threads on the windows
and belly band. Both are extremely common leak sources, pretty much both will leak unless repaired.
You will also find exposed plywood under the kitchen cabinets and under the gaucho too, if you have a front gaucho. Rot in those areas is worse.
Two layers of fiberglass would not support the weight
of someone standing on the floor. Many assume the floor is fiberglass. It's not. The floor gets its strength from the plywood. Most fiberglass trailers use plywood or OSB for this. Some have the wood sandwiched between layers of fiberglass, some don't. Being sandwiched protects it more, but makes it a lot harder to repair/replace. People also think molded FG trailers are indestructible. They are not. They just are a lot better than their stick built cousins.
Trilliums are great, but have some well known flaws. Left unfixed, they can ruin the trailer.
Go to the manufacturers section top of the page on the right, and read up on belly band repair and window repair. Both have been covered in depth many times. Lots of good info out there. There are more issues too, like body to frame bolts, and the door hinge attachment.
I rejected the first two 4500s I looked at, one had standing water in the dinette benches (it was raining), the other the wood in the dinette bench was totally dry rotted. Both had soft spots in main floor (rotten plywood). Both trailers were quickly sold
to others who I guess didn't look very close. I'm waiting for those two buyers to appear here: "How do I fix this?" I haven't seen either yet.
Water inside a dinette bench is something to be addressed quickly. Don't wait, get it dry, consider some small (1/4 inch or less) holes in the pontoons. Hopefully the plywood is still OK.