Being registered as a motor home isn't the same as being titled as a motor home. The GVWR falling into the commercial category can also be an issue.
It's all based on the VIN. Insurance companies run the VIN to see what their going to cover. If the custom built camper on the back is a DIY effort you'll will be hard pressed to get coverage, it isn't impossible, just problematic. Insurance underwriters have problems assigning a value to DIY builds.
One can buy a $250,000 Earthcruser or an old RV, gut out and rebuild the interior and have no problems sourcing insurance. Do a DIY conversion and no one wants to touch it.
Using an independent agent not tied to a specific insurance company may be your best bet. If the vehicle isn't titled as a motor home, you might try insuring it for what it is, a commercial rated vehicle for personal use. You may have to produce receipts for the DIY conversion to assign a value to the build. You may have to produce inside and outside pictures of the vehicle.
You may also be able to use "collector car" insurance. You set terms like mileage, value, replacement cost, gap coverage, and one important part of this is that you and the insurance company "agree" on terms, price and coverage or you don't buy it. No surprises. They deal with a lot of custom work and one-of-a-kind vehicles. Maybe an option.
I am currently insured with State Farm. My concern is that it is standard commercial vehicle insurance as I use the vehicle for business some of the time. It does not account for the fact that the vehicle has appreciated significantly since purchased and built. It was manufactured by Phoenix Custom Pop Ups, but has been added to and upgraded since. Progressive and most others did not want to touch it because of the combination of commercial and specialty vehicle. I have some new feelers out there and hope I get some where with an independent agent.