I have completely rebuilt several
fiberglass RV’s. And I agree that you should ensure the floor is solid before the interior build. Carefully check the frame; it may need to be sanded/finished. Check all the welds and look carefully for any cracks or bends. The
axle must be inspected, and be sure to replace the wheel bearings; start fresh there if the
axle is good. Do a leak test. Observe the
fiberglass shell for cracks or chips and see if it needs gel coat attention. Eventually you’ll need to give her a wax and a buff. I would inspect every hull penetration. Plan to remove
windows and re-seal with butyl tape, etc. The door on these is notorious for sagging. Although I can’t see the trailer, it sounds like the roof support system is gone. Plan to build-in some kind of floor-to-ceiling structure - shelving or brackets - that will prevent the top from sagging down. In the meantime, search for as much visual inspiration online while you plan-out the build. I think you’ll enjoy the process and your mother will love and enjoy the result. Good luck!