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.
I just signed up with Nationwide, who also carries my car and house. I have never had a problem with them covering any other claims so I assume they would not hassle me over any problems that I could have in the future with the Travel trailer that I just bought. The coverage is for comp/Collision only with a $250.00 deductible for collision and no deductible for comp. It is $62.00 /year. I thought that was a good deal but maybe I am wrong.
It's a reasonable price but you need to look closely at the coverage. Policies from auto insurers are often geared towards utility trailers and sometimes exclude things that are common on RVs, e.g. electronic equipment, awnings, any personal property onboard (contents), possibly even the appliances (the "house" stuff). Some of these things might be covered if in your car but perhaps not if in/on a towed trailer. Ask specific questions and take notes about the answers, including the name of the person answering. You also need to ask your agent about how the coverage applies (if at all) when the trailer is not actually being towed, i.e. set up in a campground or stored somewhere in off-season. For example, suppose there is a fire in the RV while in a campground.
I'm not saying you are not covered for any and all of these things, but it is something you should find out about BEFORE you have a claim. Some people have been unpleasantly surprised. The other advantage of asking your agent to describe these coverages to you is that, should he omit to tell you about any exclusion that later costs you money out of pocket, you will have a basis for a claim under the agency's own "errors and omissions" insurance.
Since you own a house and have it insured, it is probable that some of your home owners coverage will also applly to the RV in certain circumstances. You probably have some general liability coverage under that and it may also cover at least some of your personal property while away from home. Ask you agent about how that ties in with your trailer usage.