You need to consider that none of the fiberglass trailer are designed for off-road use. The steel chassis's and elevator bolts used to bolt the fiberglass shell to the chassis under all fiberglass trailers with the exception of some of the most recent new trailer offerings are marginal at best and downright rotten when old.
If going with an older fiberglass trailer expect to remove the shell from the chassis to upgrade or replace the chassis with a better designed and constructed chassis and new mounting fastener's for the shell to chassis. Your conversion idea can most certainly be completed with some thought and fabrication skills. Given the cost of many of the "Expo Trailer" offerings in the market today I'm not surprised that more and more folks will begin to convert fiberglass shell trailers.
yourself to inspiration from another "Overlander" who posted this Uhaul
camper build on Expeditionportal.com. He transformed his 13 ft Uhaul
CT13 camper into a camper they pull off-road and to campgrounds. The trailer has a modified chassis raised axle
for ground clearance along with exo-skeleton to protect the trailer and allow the mounting of Kayaks and other items.