Yes and no. The price should reflect the mileage and you should consider that you may need to or want to rebuild soon. It should be nice enough that you're willing to do that.
I would think arranging with the owner to take it for an engine compression test might be in order-as I wish I'd done with two different vehicles I've bought in the past.
Instead, I wound up buying a used engine during a trip. IT was down in Texas and I had a used V8 put in my 1979 chevy for $800. It wasn't a rebuild, it was pulled from a wreck, and I ran it for 6 years with no problems. The downside is we lived in a tent for a week while the work was done, but the van cost me $550 to begin with so all in all it worked out.
You should also make sure the tranny is solid. You should feel it shift if you're on it (not sure how true this is in an RV) and not be slushy when changing gears.