You asked two entirely different questions:
1. What kind?
2. What is cheapest?
And received answers mostly about what brand (company).
You also didn't state whether the RV is motorized or towable - that makes a huge difference. Motorhomes require liability insurance in all 50 states, and that's a big part of the answer to both type and cost.
The cheapest is none if a towable and minimum liability only for motorized. But if you want collision and comprehensive coverage (pays for damage to your RV) for either one, you make it cheaper in two ways: (1) insure for Actual Cash Value (ACV) rather than Agreed Value, and (2) have a higher Deductible.
The rest of the answer is that you have to shop around yourself, because premium costs vary tremendously based on a wide variety of factors, starting with your physical location (zip code) and carrying on through age, sex, marital status, credit rating, driving record, and the inevitable "etc.". No one company has the "best rates" or "best coverage" for everybody.