NOT an RV expert or even that smart sometimes :)
there is this stuff one can apply to roofs, not a great fix but may get you through...
It's thick like cake frosting, the kind I've applied to an old shed was white, laid it on about 1/2" thick. Was a little like spray foam when dried.
It'd last for the winter but so would a tarp, but IF one wanted to use the camper it may do the trick, even with less than stellar structure under it.
Ames snow seal Premium roof coatings, True value has it for 35.99, 1 gal. 1 gal does 100 SF.
I just did my own home made trusses and then applied a EDPM roof, time consuming and I'm out ~$500 total for the rubber, primer, glue and wood for trusses and 1/4 ply for roof. Of course one would want to ensure the wall wood was stong then strip off old roof, attach trusses to side walls insulate, puddy screw holes and sand apply, primer to wood. then the gluing down of EDPM.. watch lots of Videos... :)
Happy camping.
