I worked for a local dealer and consignment shop for a while and we ENCOURAGED folks to have the coach inspected at a shop of their choosing. We offered them a list of the local specialists for motorhomes including the local Cat and Cummins shops.
The one caveat was we expected an offer and an earnest money deposit, fully refundable, prior to undertaking the inspections. The deposit was intended to weed out non-serious tirekickers.
One thing we did allow prior to deposit was an non-destructive on-site RV Survey by a person of the buyer's choosing. After that it was "show me the money". I think the on-site survey is a great place to start as a good surveyor should be able to give a good overall evaluation of the coach in order to deternine if it's worth pursuing further.
I felt it was a fair policy and one that seemed to work well for us. The dealership was family owned and took pride in their service and reputation. A reputable dealer knows any disatisfaction you have with the coach will come back to haunt them. The inspections always worked in our favor as any problems could be worked out prior to delivery.
This having been said, it was amazing to me how many folks purchased a coach WITHOUT any outside inspections taking place!
Another thing we always promoted was a 1 year extended warranty to protect against any major failures. Although we made a few bucks on the warranty, we did feel it was a worthwhile investment (usually around $1000-$1500) when purchasing a used coach, inspections or not.