I think the reason some insurance companies charge more or do not offer the insurance is due to that they do not have the flexibility in their system.
When I bought my Freightliner I got a price from my existing insurance company. They said the Freightliner is a commercial truck. I told them, it would not be commercial truck. Their words were, when we put in that model into our computer the computer says it is a commercial truck!
I went with RV Alliance and my truck is insured as a heavy duty tow vehicle. The rate is less than ˝ of what I used to pay for my 1-ton pickup!
While I pay more for my license plate fees each year, I save over $600 a year on insurance on my HDT.