If you have a loan on your RV, then you will have to have insurance. The dealer is correct in not letting you drive off the lot, especially if you have a loan.
And yes, when you purchase a (new to you) trailer, you need to contact your insurance agent and get a policy rider initiated on your current policy before you pull off the lot.
I don't think the dealership was trying to scam you. He was trying to protect you. Otherwise, you could have let the RV sit on his lot another day until you got your own insurance settled.
Sounds like you got it all worked out after all. I don't think he was doing anything to pad his own pocket. And yes, it is the same as driving a car off the dealers lot.