Why?
There are no recognized standards for an RV inspector and his opinion has no guaranty. Much like a home inspector the quality you get is a toss up and if something is missed Oh Well to bad so sad.
First find a good mobile RV mechanic to come look the rig over with you. Then Find the manufacturer of the motor/chassis and take it there for an engine power train inspection.
If you choose wisely you will find 2 places to service your rig for you.
Find the mobile mechanic by calling a local big RV resort/campground and ask who the recommend.
You can also try RVservicereviews.com to find someone.
Additionally go to the local flying j and buy one of the oil analysis kits they sell. Put the oil in the container ship it off and get a report back that will be pretty thorough on the engine condition.
The last person I would use is a inspection service
Traveling with my best friend my wife!