The structural integrity of your trailer has been irrevocably damaged. The original wall panels were made with a plywood core bonded on each side to fiberglass FRP construction. Water leaks have rotted the core material and debonded the skins. The walls now have no strength and can be easily damaged by any striking of their surfaces.

The proper repair is very labour intensive, and if not effected by a DIYer, very expensive to hire. Basically the best skin inside or out is selected and left intact. The other skin is cut off preferably in a large sheet and removed intact from the core. A new core is bonded to the remaining skin, the cut off pieces are then bonded to the new core and strengthened. The cut seams are covered and faired and if outside, the panels are resprayed.

It may be easier to build new walls within the old. The inside skin should be removed, the rotted wood removed and the walls' structure examined. You may need new metal studs, stringers or other structural members. Once strengthened, the walls' inside surface can then be replaced with a variety of new materials.

You must ask yourself if a renovation of this magnitude is economically viable, considering a restoration like this could easily cost a couple of thousand dollars to have it accomplished. If you are handy with fiberglass and wood materials, the cost could be a quarter of the "professional" expenses.

I would sell it "as is" and look for something that would be safe to use.