We ran into a similar issue, and it's apparently a common one when renting. Our auto policy would cover the rented TV as normal rental cars are covered, our umbrella liability policy would cover the TT for liability, but we were left in the cold for collision/comprehensive on the TT.
If you find that you're unable to resolve this with your insurance company, you do have an option. Outdoorsy.com is an RV rental network similar to Airbnb. They actually provide insurance for both vehicles, with a $1,500 deductible. There are quite a few ASs listed, especially in the western states. Many are very new, some owners (rentees?) have tow vehicles available as well.
Finally, there are several places that rent them in place if just trying out living in the AS is what you're wanting to accomplish. One near Santa Barbara and also one near Sante Fe NM immediately come to mind. I also think there is a campground near Joshua tree that has a variety of TTs including an AS or 2. Fell free to PM me if you want more details. We've done a TON of research on this.
We are renting a TV and vintage AS through Outdoorsy next week in MD, and camping in VA as a test before we buy. So far our experience has been great; the training (we've not towed in MANY years) and administrative side of the rental has gone very smoothly. We're very excited to take this next step toward ownership!
Hope it all goes well for you!