Since you are a mechanic, you know how hard it is to judge a vehicle sight unseen.
The price is about right for an AVERAGE condition 1995 American Eagle according to nadaguides.com.
Things not mentioned are the tires, which could add $3500, batteries $1000, engine radiator $7000, condition of the AC units, furnace, appliances, TV's(old analog or more modern digital), furniture, mattresses, water pump, etc.
On the mechanical issues, the suspension has 90,000 miles so what is the condition of the shocks, and other chassis components? What about the generator? Condition of any awnings? Exterior paint?
Any water damage that has destroyed the wood floor structure? Dual pane windows that are fogged? Roof leaks and stained ceilings or possibly roof structure damaged?
He bought it as a fixer upper meaning that it needed some work. What all has he done and does he have receipts for the work? Did he ever use it on the road?
These are just a few items that you need to be aware of. It may be a great deal but it also may be a money pit. Items like cracks in the fiberglass do not repair themselves and they only get worse. Repair of the cracks and painting can get expensive.
Since he is asking about the average retail for this coach, you need to remember that any work and money that you pour in to it will not be able to be recouped if you sell it later. You need to go look at it, examine everything, and let your wallet be your guide.