Gene, Onan recommends that the genny be exercised for 2 hours every month with a load to keep it in tip top condition. That means 24 hours a year or on a 12 year old rig the genny should have 288 hours on it. They obviously haven't exercised it and it probably will have problems.
I think you made good choices to pass on the first 2. Today's RVs should not need a tune up with only 15,000 miles on it. 75,000-100,000 is more like it.
Also check the date codes on the tires on any you are considering. RVs generally don't wear out tires but they will crack in the sidewall and be useless after 6-8 years regardless of the tread. On a Class C, budget $1,200-1,500 for new tires.
After you determine you have found one that you really like, then spring for an inspection by an RV shop. It cost me $250 for a complete written report for a house and chassis inspection. It was well worth it for my peace of mind. They did find a couple minor issues that I did not catch so I could consider that when we made the final deal. Good luck and enjoy the hunt!