Did the odometers then read full mileage, or reset to zero after 99,999.9 miles? If it's the latter, it could easily be 100k or 200k over what it's showing.
Generator is probably an Onan or a Generac. I don't know if a newer one will fit, but someone skilled (read high $$/hr.) needs to troubleshoot it first. Many generator issues are caused by disuse, and fixes can start about $300 and go up from there. Replacing with a new one, if possible & simple, will run $2.5K > $3.5K, maybe a bit more. A cheaper but hasslesome alternative is a portable on a cargo rack.
Tires should be not over 6 years old, regardless of tread remaining. There are articles on the net on how to read the date code. They're probably 16.5", of which very few varieties are made any more, and freshly-made ones are hard to find.
Check _thoroughly_ for soft spots in walls, ceiling, & overcab; rot caused by water leaks is the death of _many_ class C's.
Finally, if you're in a northern clime, best forget 'living' in it. Cost to heat would be about as bad as rent, condensation & resulting mold might be unhealthy, and water system is very unlikely to be 4-seasons. _Not_ for the unskilled RV'er.
Jim, "Mo' coffee!"