Your camper is in a grey area. No valid VIN, so it's kinda viewed by most insurance companies as a "camper shell" or accessory. A camper shell (and accessories) is/are covered by the TV's insurance up to a set limit specified in the policy---as long as it is attached to your TV. If your truck camper is at or under that limit then you should be good. If it's higher, then IMO your only option is to establish a new, valid VIN (following procedures defined by your state of residence) and insure it through Progressive, GEICO, etc.
I believe you just rebuilt your camper. In your mind you have an idea what your camper is worth after this rebuild. I think it would be prudent to make sure whatever insurance company you insure it with concurs with what you think it's worth.
Lastly, I'd take any advice an insurance agent offers to you with a grain of salt. As boring and frustrating as it is, your best recourse is to read your insurance policy. Generally speaking, unless a policy specifically excludes coverage for a given situation, the odds are somewhat in your favor that you're covered.