OK, so maybe I'll back off, just a bit. My ins agent (whom I also live with, so I know this stuff a bit more than others), says that if
1. The GVWR on the data plate is such that you need an endorsement, i.e. non-commercial class B, get the endorsement. If you don't have the endorsement, and you do something REALLY bad, like drive it into a building, kill a bunch of people, your ins co MAY be able to deny coverage. But it will usually, as I said, need to be pretty bad. Get lawyers involved... You're basically operating the vehicle without a license.
2. How the vehicle is registered is more between you and the state. As long as you have the proper license to drive the actual vehicle, the insurance co is happy.
I suspect in this case, for example you're driving a REALLY big bus (like mine!!), get pulled over for some little thing, cop sees you don't have a Class B, and checks the plate, you'll get a ticket. Unlikely? Maybe but we all get unlucky. Because IT IS WHAT IT IS, regardless of what the lady in the DMV window let you say it is. My bus has a GVWR of 32,800 lbs. I need a class B, end of story. Doesn't matter how it's registered. That's why they have requirements. "Why no, officer, I don't need that endorsement. It's a two wheel car, not a motorcycle" Yeah right.
SO.....my suggestion is to register it and insure it in the easiest and cheapest way you can, BUT make sure you are LEGALLY able to drive whatever it is you're driving, if you want to avoid any issues. And yes, I drive mine without yet having my Class B. But I'm going to get it, really...
Basically my issue is more a licensing issue, as opposed to registration. A subtle yet real difference. And remember, most ins agents are like the lady at the DMV window.....Buyer beware