Had this problem in 2010. 4 of us went, only one could get insurance. No one else's insurance would provide the "binder" on an RV. I think he had State Farm. I'm with GEICO (for a long time, and several cars, ATV, motorcycle...) and they told me no way on a rental RV. He had to do all of the driving. It was only a couple of years old, and I think they required $150,000 policy. (it was a really nice RV...)
Last year, he just bought a late 80's RV for the same price as renting for 2 years...so this will be year 2 in his RV, and since he owns it, no more insurance issues.
My wife and I did an east coast trip last year with one of the big rental companies, and they provided an insurance option. Apparently a lot of the smaller companies or individuals that rent their machines require you to get your own insurance, which can be difficult, as we found.
Good luck, I hope you find a solution. If you do, please post...
Burner from Baltimore