We have the Good Sam/Camping World/National General Insurance. Rates are reasonable, but slightly higher than my parent's RV insurance rates (through their auto carrier). If the rates go up next term, we'll re-evaluate. Much of it depends on your state and credit score, so its hard to compare from person to person on this forum.
FWIW, I do think there is some value in having an insurance carrier that is accustom to RV's in lieu of a primarily auto-based carrier. If I had a yacht, I'd probably find a company that provides insurance exclusively for yachts. But, like most folks, I hope to never have to use the insurance we have.