Well, insulation ain’t cheap. And remodeling an old and rotted travel trailer provides likely 0 or negative ROI.
Id be looking at Habitat for Humanity or Jobsite scraps of anything from open and closed cell foam to fiberglass batt.
Idk how but if you could get ahold of a truck load of shredded paper from them outfits that dispose of documents, that would work too just be more of a challenge to get the walls stuffed. I’ve thrown away enough old blankets and towels that some judicious use of a stapler could make that work too.
Point being, pricing just 1” foam for my 28’ enclosed trailer was $400-600 for eps or xps and then another $500 for some cheap white mdf sheeting.
If you have 2” walls double the insulation cost.
Min $1000-1500 for insulation and sheathing. Is it worth it? Are you going to see enough Heat or AC required days to see more value in spray foam or xps?