If YOU have the knowledge to thoroughly inspect a motor home to insure that everything is working correctly, or the knowledge to diagnose what is not working, you will be better off buying from a private party, that has kept complete records. If you read this forum you will find many instances where the dealer has not taken care of warranty repairs or instances where the coach has set at the dealer for weeks waiting for repairs, while the monthly payments go on.
If you do not have this knowledge, then it really doesn't matter too much who you chose. You take your chances and hope for the best.