Although we're from Virginia and it's not legal here, I know double towing is legal in Michigan and quite a few other states, especially west. Although I've seen triple towing semi's out west, I've never seen any RVs doing that.
First though you'd better verify your trailer's hitch is up to towing anything at all. Most of them are really not good enough for even a bicycle or two. Everyone I've seen towing a boat behind a trailer had a custom hitch welded onto their frames.