I just switched my insurance from National General to Progressive. I got NatGen insurance when I bought the bus in September 2013 and paid $1,400 / year for MD minimum liability with them. This was largely because I was 19, male, and had a speeding ticket and "unlawful use of license" ticket (caught driving after under 18 driver curfew).
Just on a whim (well, after making another monthly payment of $106 for my bus to sit, insured, in the driveway) I went to Progressive's site and saw they had an automated quote option for NON-Professional bus conversion under Motorhome insurance. I put in an estimated value of $15,000. It did not specify that it could not be a school bus. So I went through with it, put in my info, and picked the BEST coverage I could, and it is $698 per year. HUGE difference, less than half as much for better coverage. I attribute it to me being older, having no accidents in my documented history (or history ever!) and the tickets I got in 2011 were obsoleted due to them being over 3 years old.
I submitted this and paid the first month, submitted a week ago so I figure a real person has had time to look it over and they haven't rejected coverage yet, so that is a good sign that it is a real deal. Huge amount of money saved and I feel a lot safer if an accident happens!