Maybe drive the RV over to the DMV and try again. It's very possible that the original person at the DMV is just someone who is jaded with their job and didn't want to help. And the next person you talk to is more amicable and willing to help you resolve it. Even go to a different DMV office if the first one you were at talked to their manager and the manager was the not so helpful one. Explain that you have this issue, the seller is not going to help, the insurance says xxxx and please help you fix this. The RV is right there in the parking lot if they need to see it.
Or you can go the route you are saying where you get a third party involved, you spend a lot of money on those third party fees (possibly more that the RV is even worth), and eventually it gets resolved.