In Pa, IIRC, you have to pay for an extra fee for a "recreational" vehicle. (motorcycles, mobile homes etc) The first year its a bogus extra fee, but after that I think you can renew only the recreational coverage.
Double check with your local AAA office but thats the way my buddy in Philly broke it down for me based on what he has. I have my own trailer, and also have a decent network of "buddies w/trailers" so I dont worry about it as much.
hope that helps