Advice on a Class C sizes..... 26' or 31'

You might want to contact the owner of the Monaco. Most 31's in those years come with a 3500 lb hitch, the ad says it has a 5k hitch. So he may have done other things to beef up the suspension besides the air bags like shocks, sway bars, etc. It is tough to find them 10 years old+ being garaged. Since it has the original owner, he can tell you everything that has been done to it. I would definitely pursue that one if it were me. Depending how old the ad is, it could already be gone. The clean ones go quick.