Ken,
In addition to what we've already said, please realize that the answer to your question is a bit like 'how long is a piece of string'. For any given RV and insurance carrier, there are different levels of coverage such as collision (used to be called "third party insurance" in the UK) and comprehensive (UK used to call it "full comprehensive"), liability, property damage, medical payments, bodily injury, uninsured other driver and a number of other coverages. Additionally, the premium can be higher or lower depending on the "deductible" - this is the amount you agree to pay out of your pocket before the insurance company pays out.
There will be large differences in premium between insurance companies and everyone believes they have the "right" or the "best" deal. Unlike most others here, I don't shop for the cheapest quote. Rather, I insure our coach with the same company that has insured our house, cars and lives for the last 26 years - a company something like the Prudential used to be in the UK. I learned this lesson the hard way from some low cost carriers when I lived in the UK.
I pay $2,400/year for a coach that cost over $200,000. This premium is discounted by approx $3,200 for driving record, no claims and "loyalty". So, with this company, I'd be paying $5,600 without the discounts. I carry low deductibles and high liabilty coverage, both of which increase my premium significantly. Others have different priorities for these coverages and there's no "right" universal answer.
For reference, when we first came to California in 1980, the same company charged us 100% above the "normal" rate to insure our cars because of the lack of a driving history in the U.S. They reduced the rate to the "normal" level after a year of claim-free driving here.