When to throw in the towel on an older motorhome

The thing is either going to be like a codependent relative that when you "fix" one thing another thing breaks or at some point it will level off.

I'd get an inspection done now to see - what repairs will be needed, what further bugs with that model you can expect and what it's worth.

Frustration leads to burnout but it may not be easy to find another unit and the benefit of the one you have is that it's the devil you know rather than the devil you don't.