Look in the classified section of RV magazines – I saw one in the back of my MOTORHOME magazine from the Miller Insurance Company – they state that they specialize in RV insurance, and can handle a medium truck (1 ton and larger). Read the ABOUT US section, and you'll see that they have a long history with trucks.
Miller Insurance:
http://www.gomillerinsurance.com
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Another option is the guys who insure semi tractors used as RVs:
http://hhrvresource.com/node/6
The section on insurance (it's in “The HDT Decision”
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offers some hints that might help with insurance.
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Insurance is out there, but the agent's perception of you and your usage is also important. The magic word "Ambulance" can cause problems, until they understand that it is a DECOMMISSIONED ambulance, and it no longer has a siren or red/blue lights, and all the "ambulance" lettering has been removed - it is now basically just a truck. A photo can help.