There is a saying, "you can't polish a turd". I saw a show where they tried to prove that wrong. I think the show was Myth Busters or something like that. They did eventually get a bit of gloss from a round horse turd, It was not shiny. A few hours later it was dull again.
Point being that things are manufactured a certain way. A stick built is basically stapled together. Every joint and connection has some degree of movement. Over time they weaken enough to leak or worse. Covering it with a thin cost of fiberglass might end up shinny but not for long. and the added weight
of the FG may speed up that joint fatigue. The movement of those joints will result in the fiberglass cracking and possibly falling off. A trailer that has some value now, would be worthless fairly quick.
We can get a bigger sticky for the dollar than a molded trailer. As time passes the molded ones hold their value, while the stickies become very cheap.
So if we want to live at the campground with all the ammunities of home, we need a larger camper and tug. Be prepared to pay up front or pay more later on.