It depends in the type if coverage you are seeking. I was in the insurance business for 12 years (licensed general lines agent) and would advise if your RV is newer and you are wanting to really cover it for "full" coverage (meaning most everything including liability while parked and unhooked from truck...remember the time frame we live in today...it's a dog eat dog world with a lawyer on every corner), your best bet is an RV policy vs bundling it into your auto policy (the exception would be if your parent company also has an RV policy). An auto policy will not provide liability coverage unless you are connected to the tow vehicle. There are other exclusions as well but you would have to read your individual policy to know what they are. I am only familiar with Texas policies. I found there is not much premium difference between adding to your auto policy or placing on an RV policy but the coverages afforded on the RV policy far exceed those offered on the auto policy for an RV. Plus an RV policy can act just like a homeowners policy (only mobile) with respect to contents etc. Again, it really depends on the coverage you are looking for.
With all that said...if my premiums doubled at renewal with any coverage I had, I would certainly be seeking comparison quotes to see what my options were. If you use an independent agent for all your coverage, they will have other markets to get quotes from. If you go direct to the company for coverage, call them and see what else may be available and ask why rates increased so drastically? Seek additional quotes from other carriers for comparison. Mine just renewed with Progressive in May and my premium went down $3 a month (not a big decrease but it wasn't an increase so I am happy).