My 2000 17footer has a plywood floor with heavy glassing on top and underneath.
An aside about fiberglass: the white finish is gelcoat, which I think is a least a little bit porous. I will eventually paint
mine with enamel, which should be less porous and harder.
If you have chemical allergies I would say that only used trailers should be considered: We got ours at two weeks of age, and the next year was spent keeping the air circulation going to keep the smell from getting too overwhelming, and we don't even have allergies. Don't know whether the outgassing was normal or due to a resin/hardener mix which delayed curing. At this point everything is fine.