There is no single answer - it depends in a major way on the state you are insured in and then on a number of personal factors as well. You might fall into an expensive rating category with one insurer and an inexpensive rating with another.
But we too have found Explorer RV, which is an insurer owned internet agency, to provide generally excellent prices. We've been with them for 3 years now and nobody has beat them (I get quotes every year at renewal time). I suggest getting a quote there as well as at least 2 or 3 other places. You can easily get a quote through Good Sam, but it will probably be the same as your GMAC policy. Last I knew they used the same underwriter.
Some insurers do better on trailers while others may do better on motorhomes.