Take it to your trusted mechanic before you buy. If the seller isn't amenable to this, run away. If s/he is okay with that, pay for the mechanic to go over everything, and find out how much the repairs (there will be repairs) will be. (No doubt, hoses, belts, etc will need replacing, see where the rust is and get an estimate (or your mechanic's best guestimate of what a body shop would charge.)
Go over everything on the RV side too (unless you have an rv dealer who will do this for you): first thing, turn on the genny and let it run while your going over everything else (this way, you'll have time to listen to it); run water in the sinks and flush the toilet and have someone else outside lying on the ground (next to, not under) the rv to see what, if anything, is leaking as you do the water test. Also run a cup of water on suspicious areas to check for leaks.
Open the awning. Run the AC (on genny power), fridge too. pull up the carpets (if you can) and see what's underneath. Get your face down into the cushions: a cleaning can get rid of topsoil, but mold all through the cushions means replacement.
That's for a start. But don't rush just because the price is so tempting. Time now is money saved on what might be a huge headache.
