The fact that you ask the question gives the answer - if you do not really understand this kind of thing then you can get into more trouble than you can ever believe.
68,000 miles can be a lot on a big block gas engine (which will get, if in good tune, 5-6 mpg by the way). Same for the transmission. If not abused and well maintained both could last tens of thousands of miles more, but you really have no way of knowing. Was the chassis of that model up to the load and length? It may (probably, in fact) need six new $200 tires. And on, and on and on.
Also be aware that the used market for gas engine non-slide motorhomes has just about dissappeared. I note that with one hour to go, there are no bids.
'85 Sovereign, 25'