Thanks to this thread, I just got a policy from National General - full-timer Class C RV policy, 12 months, for less than $1k. Not too bad!
I tried various other insurance companies, but they all fell apart for various reasons. Progressive mandates that you have a second vehicle that you tow, or else they won't insure an RV full-time - which is kind of absurd and defeats the whole point for me. A couple of other companies won't let you go any further in quote generation without providing a garaging address, which again is kind of antithetical to the idea of full-timing in an RV. And any company that can't give me a quote online doesn't get my business - I was surprised how many of the RV specialist companies mentioned up-thread require you to call.
I'm a total newbie to RV-ing, and I didn't expect it to be so challenging to get an RV policy, so thanks for all the input in this thread. Surely RV-ing full-time without a permanent garaging address and no other vehicle can't be that unusual!