Sometimes with a dealer you can get a warranty, even if it is for a short time.
Buying from a private party can save you some money but you lose any warranty advantages.
The main thing is to look at the service records. If it was serviced by a dealer, the records will show this for you to see that the coach has received regular maintenance.
From a private owner, you still may be able to see service records, if he took it to a shop for maintenance.
If the owner did his own service, you have to trust that he did it right and that the paper work he is showing you is truthful. Anyone can write a log that shows the oil being changed regularly and the same goes for the rest of the recommended maintenance.
Buying from either has certain benefits but I would only believe service records from a commercial shop. This is coming from a guy that has always done all of his own RV and other vehicle service, except for warranty or recall service.