I'd like to echo John's comments.
We have a 1991 Scamp 16 and the floor is perfectly fine after 22 years. We find the thickness of the wooden floor perfectly adequate and note no deflection as we walk about.
As to going to a heavier guage tubing, I don't know why. Ours has stood up real well and is heavily used. Another option would be judicious stiffening of the frame, an easy and relatively inexpensive posswibility.
We have painted the top surface of our floor where it was easy to do, in cabinets and under part of the carpet in the dinette area. When we had a recent accidental spill the paint
kept water from being absorbed into the floor where there was paint
. No damage was done because we quickly dried up the spill but you could see the difference between painted and unpainted areas.
If I had extra money to spend I would reserve it for items that make the trailer more useful and comfortable.
In closing Fiberglass trailers are amazing in that they truly maintain their value, our son has a 36 year old Scamp 13.