I added my 4WC Hawk to the insurance on my 2005 Tundra. Mercury was no extra cost for adding it. I asked about value to be sure it was covered and they told me it did not matter what I paid they would only cover for fair market $7,000. I am a happy camper twice what I paid.
Register/insure the truck as a "truck", then add a rider on the policy for the camper.
The rider for my FWC (which is much older & not nearly as expensive) runs about $40/year, & is tied to the truck policy (IE: no insurance if it is on a different truck).