Clearly, some folks didn't look at the ad ;-)
We own a 2004 Gulf Stream Conquest and have been crazy-happy with it. Had many wonderful sightseeing trips in it. Now, we are trying to spend more time in one place, so have gone to a 5th wheel with a big slide. The slide sure does make a "livability" difference!
*IF* the slide leaks, you should see discoloration in the flooring or bubbling of the wallpaper near it. The floor might feel "spongy". There might be bubbles on the outside fiberglass, which is a sign of delamination. Most slides have multiple seals, though, and are pretty watertight. The only thing that would concern me, is if the MH has the smaller F-350 chassis? The slide adds about #1000; make sure you have at least #1500 cargo carrying capacity. It's amazing how fast the "stuff" you bring with you can add up in weight.
Ask where it was stored - if it lived outside - plan to re-seal all the windows, vents and lights just as a precautionary thing.
Looks like it was somebody's "baby", though - and that's what you want!!