My experience. Most of the insurance brokers I talked to (and I live in the Northwest) really didn't understand truck campers. I got the: "covered under your auto policy when on the truck - its cargo, homeowners when off" story from some. I asked them if I transported a $75,000 dollar grand piano would it be covered and they started to back down. One broker tried to cover it as a separate rider as a travel trailer. When I called PEMCO the broker instantly knew what I was talking about and their computer had a policy form right on the "pull down" menu. My policy explicitly says "truck camper", names the brand, make, serial # value etc just like a car.
If you read through posts you will see some people have had good claim experience with the "covered by this on truck and this off the truck" policy and others have not. My suggestion is to be sure it is explicitly stated, in writing, on your policy what your coverage is.
I personally feel much better have it explicitly covered.