If your trailer is located in the New England states, a "canvas and PVC" roof is not going to hold up to the snow loads you get and will fail catastrophically and spectacular way at the worst time. For a very "temp" fix you could tarp the roof just to keep anymore water from coming in but that is a very short term fix for a few weeks to a month at best.
Conventional roofing with shingles would be pretty heavy but it is possible that a pitched steel roof will be light weight enough to directly placed on the walls. Depending on your area's building codes, steel roofs typically will use 4ft centers and purlins spaced at 2ft. Which will be far lighter weight than conventional shingled roofing and should not require any framing to the ground.
No matter what you do, you will be spending far more money on this trailer than what it is worth and you might want to look at the idea of finding a newer and in better shape trailer. You have a 30yr old RV, basically worth not much more than the sum of all the scrap prices of the parts..