It's a much bigger hassle, but if you make multiple trips or do stuff via mail you can get the title signed over (and pay the seller, etc.) in Florida, take the title and paperwork home to West Virginia, and get your registration etc. there and then move the RV.
Florida collects tax on the in-transit permit, which I think is somewhat unusual--the few other states I'm familiar with just have a flat fee. Many states allow offsetting of tax paid elsewhere, but not West Virginia and a couple others. It seems there's no way around the double taxation with the in-transit permit.
I would not drive it under the previous owner's registration and insurance. If I were a seller, I would definitely not want the buyer to do that under my registration and insurance after it's sold, too.