There is so much information on the Majestic 23A in the thread IAMICHABOD referred to, as well as other Cruise America models, that I highly recommend you read. The thread should put most of your fears of buying a used rv with plenty of miles on them to rest.
My 23A from CA had 128,000 on the odometer and I haven't had any problem worth mentioning. I did replace the shocks and steering stabilizer on it as my first major purchase, and I would recommend you do the same. I also added a wireless backup camera because you have a huge blind spot behind the unit.
re: driving on gravel roads-
You're driving a big, overweight vehicle on a rough surface so take you time and avoid thinking it's capable of off-road use. The biggest problem I've encountered is dust getting into the unit. How the unit is attached to the subframe leaves a lot of gaps that the dust will find a way into. I've spent plenty of time underneath the unit plugging these gaps with caulking, expanding foam, and spray on truck bed liner. I've eliminated most of the gaps and the dust incursion is almost nil. This is a fairly typical problem of most rv's, but I think it's time well spent.
The 23A is a base level RV, and I'm good with that. I have no problems recommending a former CA rental to anyone, the miles on the unit don't bother me. The engine and chassis on these units are good for 1/2 million miles if they are maintained, and in my opinion, the rental companies do a better job of maintaining them than a private owner.