Foremost Insurance Company (800-237-2060) has insured my Mitsubishi FM260 camper for the past 8 years.
Foremost would be the place to start. Pictures and documented cost of big ticket items will help assign a value to the build. Any receipts for professionally done work, like mounting the body to the truck will work in your favor.
Not sure how California does it, in Missouri, an RV certification requires a vehicle title change and a new VIN assigned. The paperwork is handled though the DMV office. But the RV certification process and the application of a new VIN plate is handled though Missouri's State Highway Patrol Inspection Station by appointment. It isn't as simple as a change in registration documentation. The vehicle's new VIN number will identity the vehicle as an RV when an insurance company runs it.
Let us know how its works out. Getting insurance for these trucks is not impossible, just problematic. It starts with a denial because the truck's GVWR makes it commercial. Then your response is that your doing an RV conversion, which leads to more denials. Try insuring it for what it is, a commercial truck for non-commercial personal use. Read the fine print on commercial truck polices, often there's a radius limit placed on the truck, I've seen it as low as 25 miles from the address on the policy.
One more thing, make sure everyone is using the correct VIN number, if its off by a single letter or number, it could push the GVWR up.
The
Tiny House movement has had its share of insurance issues as well.
