I have state farm on my mdt toter. When I bought it, it was just a cab chassis. Told my (long time) agent what I was building, he kind of scratched his head, called it a box truck, and assured me that any attached trailers were covered under the truck liability policy (full trailer coverage is a separate policy of course). My premium is about $600 per half, and is also rated for business use as I originally pulled a 40' race trailer for my business with the truck. Now we only use it for the 5th wheel camper anyway. Getting them to cover the actual value of a custom built toter may be trickier, in my case the truck is not that expensive, and if they write me a check for what a box truck is worth in the event of a total loss I would be satisfied. In my case not worth the hassle of a stated value type policy, might be in your case.
Separate note: as a car guy myself, I am betting that at times you are transporting cars that you are going to put up for sale at a show, or cars you just bought at a show to resell, or cars for other folks (even if you are hauling for a buddy). This could certainly be construed as a business use of the vehicle in the event of an insurance company trying to deny a claim, so I would make sure you are covered for business use with your insurer. I know in my case it was not that much extra. This should not affect your dot rules unless you do have a legitimate business related to your cars (in which case you need usdot numbers anyway), you will just be sure to tell the friendly dot officer you are a hobbyist in a registered rv, and not a commercial truck. They don't check the details of your insurance.