American Family has standalone ins for our TC and boat/trailer so I'd imagine they do other towables too.
Bottom line just shop around and find the best rate for the coverage you want.
I don't believe for a second that the "service" offered from one company to he next is repeatably better than others. Based on positive and negative experiences with a host of "premium" companies. From great service to outright fraud.
One thing is, it pays to shop around every so often, if you get a sniff of your rates going up without a material reason (at fault accident or something). Insurance companies are no different than your Cable TV or cell phone service, except that their "introductory" rates last longer usually and you don't know going into it when the great rate will end.
Last one was Progressive, had them for all the stuff with wheels for quite a while prob more than 10 years and several different states.
2 years ago, went to add a new car, got a quote on the whole policy, seemed like minor increase across the board, then the 6 month renewal hit a month later and everything jumped up by like 25-30% (no claims tickets accidents etc to cause it). Was told it's simply a market adjustment.
Got a new policy for about $800 less a year somewhere else, same or better coverage.