“is it legal to license and insure the RV in MN while being a resident of FL?”
No, otherwise the state with the lowest fees would have 125,000,000+ registrations. License fees raise money for states. Florida would come after you...you are a Florida resident. Next, not all insurance companies are licensed in every state for the sale of insurance nor do they wish to defend you in all states. They are hoping any accident you have is in the state where you reside and they have captive law firms. Statistics show you’ll usually have an accident near home.