I also use National Interstate, but have only been with them for less then 2 years. Got it through Miller's in Oregon. As someone else stated Miller's is very RV friendly and is an independent agency.
Make sure you tell the insurance company that you are full-timers. It makes a difference. If you don't, you could get quoted the wrong rate and maybe find out you're not covered if you have a claim.
The difference is that your rig is used all the time when full-timing as opposed to maybe just weekends during the summer and fall and the rest of the time, it's in storage. So it's logical the rate will be higher.