It is always a******shoot on anything used. Washing machine or jet aircraft. Consider private sales as you can usually judge a rig by the person who's selling it. Consider oil analysis for both engine and transmission. The cost is $25 per test. Don't get too hung up on mileage. Better to have a well maintained coach at 100k then one with 5k and no service in 10 years.
Lastly consider a service contract, if your not a diy person it's worth a thought. Research is must on these but even just a drive train one is worth a look.