As stated above I agree that it's probably a bad idea.
Fiberglass "Egg" trailers such as Casita/Scamp have a wood floor that is sandwiched between 2 layers of fiberglass. They are then mounted/attached to weak light duty chassis's through that fiberglass/wood floor sandwich with "Elevator Bolts" that easily rust and or can snap. Much of the upper shell structure when assembled comes from the fiberglass cabinetry within. Remove that limited cabinetry and you have a real Wet Noodle of a structure. The shells are typically molded in 2 pieces with the bonding of the 2 pieces around the middle of the shell (Belly Band) or vertically for older "Egg" trailers. Rarely are the shells more than a 1/4 inch in thickness. Door hinges and door sag is always a problem with these trailers along with roof sag over time. Stick a flashlight up to the exterior of the shell and have a look inside to really see how THIN the molded shell structure really is.
I LIKE FIBERGLASS SHELL TRAILERS for their intended use and have owned many different makes and models. The newer the fiberglass trailer the better the construction of the shell however the fiberglass shells are still not designed for off-road use at all. You can easily shake one to pieces!
There are some folks who have tried using fiberglass egg trailers for an Overland trailer with some success after heavy modification due to the light duty nature of the chassis and fiberglass shell. About all they report is success with Forrest Service road travel and not much more.
You would be far better off finding an out of service Ambulance box to mount on your 5 ton truck. Structurally built to withstand a rollover accident an ambulance box will stand up to the pounding of off-road use mounted on a 5 ton truck.
I have attached a picture of a modified old Uhaul CT-13 trailer the owner uses for some light duty off-road camping. You still have the issue of the fiberglass shell bolted to the frame through the fiberglass/wood floor structure with Elevator Bolts trying to keep the whole shell mounted to the frame. The "Exo-Skeleton" structure does help with frame rigidity however placing all that weight up high while useful for space and cargo hauling does not help overall balance at all.