You can remove Progressive from the list above.
They will NOT insure a self-built camper-van/RV/motorhome, according to what I was told by a company rep and the website itself. I was told that having even an air mattress or folding cot inside makes a cargo van un-insurable! (idiots).
She did say that IF the van came with factory passenger seats in the back, as some do, then it's a passenger van and they are fine with that.
I was told by State Farm (the agent called the home office while I was sitting there) that they WILL insure a self-built camper-van/RV/motorhome IF you let them look at it and make any changes they require, AND get it re-certified as a RV/motorhome by your state DMV. (or whoever does that in your home state). I was also told that during the build, from purchase to completion, they will insure a cargo van, boxvan, etc, as a commercial vehicle.
https://www.statefarm.com/insurance/sport-leisure-vehicles/motorhomes
If you have an office nearby, it might be worth a visit to verify this information with a local agent.
I also called AIS and they declined to cover a self-built RV of any type unless it is a bus conversion, and only a passenger bus. However, school buses, shuttle buses, and transit buses need not apply.