The market for old coaches has always been very bad.
There are two things you have to look at:
Has it ever had water leaks? This can destroy the structure of most RVs. If it has, walk away.
Do you know what chassis it is on? If parts are still available, how expensive will they be?
The appliances are all so old that if they sneeze, they will probably have to be replaced. A reefer is 1200$+, furnace is 600$, stove - 500$, waterheater 300$ and a generator - no telling just get out the ductape. This is just the replacement cost, no the install labor.
I was once give a 1979 Cayo Motorvator that had brake trouble and had not been driven for a few years.
I have seen old but driveable coaches go for 1~2k$.
After you find out what does work,
Make an offer that leaves you enough money to fix what you did not find.
A lifelong waterman and his bride going dry places for as long as the fuel money lasts.