Eternabond is the best patch solution but you'll need to clean the area to hospital grade. If the rig is in the woods, it probably has a good skin of algae and maybe, moss on it.
The easiest cleaning solution for me is Jasco Prepaint Conditioner. It is a phosphate replacement product but it eliminates oxidation just like TSP. I apply it with a garden sprayer, wait, maybe brush, then respray and rinse.
Elastomeric latex roof paint is what I'd use for your budget. I sprayed two coats on my aluminum roofed trailer that is stored outside, six years ago. It is dirty right now (also has a bit of snow) but is hanging in there well.
BTW, a leaking RV roof is the #1 cause of early death, IMO.