This really is a hard question to answer on an older coach, a lot depends on how it was maintained, how much deferred maintenance will you have to deal with (hoses, bushings, belts, fluids, roof sealant, ...), how many of the major appliances have already been replaced ($1,500 for an RV refrigerator, $1,000+ for a roof top air conditioner, ...)
Many of these things do not depend on the mileage, the engine and drivetrain if maintained properly on a motorhome almost never wear out, the problem instead if often that 20 year old coach that still has he original nasty transmission fluid in it because it only has 45,000 miles on the odometer that causes a $5,000 transmission to fail.