You can't compare premium rates unless you are getting the same coverage. The premium for a 5th wheel trailer worth $40,000 or $50,000 won't be the same as coverage for a Class A diesel pusher worth $250,000 to $350,000. Also what are the coverage limits? 50,000, $100,000, $250,000? What is the deductible? $100, $250, $500, $1,000. All of these variables make a huge difference.
As far as insurance companies are concerned State Farm writes the most policies of all of them. This includes, auto, home, rv, boat etc. Foremost is a division of Farmers and is good on recreational vehicles as is Progressive. USAA, Gieco are also good. Safeco, Allied (a division of Nationwide) are good too. AARP is broker for Hartford, they are not bad either. The question is not just the premium but will they pay a claim? All of the above are reputable companies. All will offer multiple policy discounts if you have your auto, rv , and home insured with them.