You can, but may I ask why you would want to do that? Yes, it may cost slightly less than normal auto insurance, but that is because RV insurance does not cover you for every day iife. It is intended ONLY for the couple weekends a year your RV leaves the storage facility. Running the get groceries? Not covered. Going to your bank? Not covered. Get what I mean? You only only covered during "recreational travel" (too and from a campground).
To answer your question though, normally if your van is registered as an RV, your insurance company will insure it as an RV. To get it registered as an RV, talk with your local DMV. Each state will have different requirements, but normally if the vehicle has a bed, toilet, kitchen facilities then they may consider it an RV.
Keep in mind though, a registered RV can not park in many places in many cities now. Can now stay parked for multiple days in other places. And generally come with a different bag of restrictions than a normal auto classification. And honestly, it doesn't cost that much less.