At what point do you call it quits on repairs??

With all the modern and effective roof treatments available for roofs these days, I'm sure there's something out there that would be in your price range.

I think POR15 is the kind of stuff I'd look at to coat the roof with. And then Eternabond tape to tape up all the seams around vents and front and rear end caps. Or side seams.

I'd say stick with it! It's a hobby and you can't put a price on how much enjoyment a hobby gives you. No RV will ever be a bargain in the long run.