For any policy, car or RV,
you will need a permanent address. However, I wouldn't recommend using an address in Michigan.
Because of Michigan's DMV laws, they are the most expensive state in the nation to get auto insurance from. Although RV and auto insurance are completely separate, this could be why you're getting such a high rate. I would also recommend companies like Foremost,
who specialize in RV and mobile home insurance. Best of luck. Here is more information regarding getting insurance without an SSN