This is going to sound odd - but do the sellers live on a gravel road? The reason I ask (and I'm going to assume that NY is similar to Michigan): anyone who lives where snowy/icy roads are treated with salt, is going to have rust. Most of our vehicles endure this very well, actually.
The popularity of Hydraulic Fracturing (fracking), has oil/gas drillers needing to dispose of the waste fracking fluid....and since it contains salt, it's been getting used to keep dust down on gravel roads. It's commonly called "well brine". The problem is that it contains more than salt - quite often it contains acids, that help break open the rock the drillers are trying to go through. Here in Michigan, a few townships have banned the use of well brine, because vehicles and farm machinery seem to be rusting out a lot faster than normal. We have a used-car lot, and have seen what drilling-construction site trucks look like: I can attest to the level of damage (to the point of structural failure) brine can do.
So it's just something to put in the back of your mind as you look at the RV....