So Giz, you carry commercial insurance on your work vehicle?
To the OP I know some states require commercial license plates on a commercial vehicle. Has to do with wear and tear on the roadways so they want you to pay extra for your plates along with commercial insurance with a higher liability, etc. Check your state DMV.
I found out quickly most companies don't offer policies on DIY conversions. Some are ok with you making changes to what's inside a Van. Add water, toilet, cooking device, ?? , then getting it titled as an RV. Then you can ask for insurance on your RV. Even then you may need a prayer to make that happen.
If your vehicle is paid for all you need is liability insurance so much cheaper and just insure as a van. Don't mention the inside stuff. If anything get stolen your SOL.
I know I considered buying an old 22' RV and throwing money into it just because of insurance. I was about to go that route when my insurance gal suggested a u-haul and build it anyway I wanted and get liability only. It was/is titled as a Commercial vehicle, now for "personal use" so no commercial plates required.
Also "Escapees" RV Club may be able to help too. Everything's worth a try.