There won't be any listing on there. They are pushing GMAC, but they underwrite through other companies as well. Just like Allied. You have to call them to get the quote and ask for quotes through other companies. They will do it. At least they will with Allied. We've had it since 2010. Even have it on our golf cart through Allied.
If you don't want to call the Good Sam Insurance and ask about it, then go to the Allied Inusrance website and inquire there. RV's are listed under Powersports. alliedinsurance.com . I'm paying right at $500/year (including the surcharges for making in 4 payments and not all at once). I have my decutibles set at $500 each. And this is on a brand new motorhome ($75k). This is only about $150 more a year than what was on my TT, that was valued at about 1/3rd the price.
The quote from the same person (during the same phone call) but through GMAC (who they automatically went to) was right at under $1000/year for the same coverage and same deductibles.