I won't weigh in on the legal requirements. I have no idea what individual states might require. It does however seem like a good idea to get lights at the back of the toad.
You didn't indicate what kind of wiring plug you have installed for the dolly. I've seen lots of flat 4-pin Y splitters. They are used a lot in pickups where there is a light bar in addition to a trailer.
I think 6 or 7 pin round Ys are going to be hard to find. One option would be to make your own by installing a second socket on the back of the RV or on the dolly itself. You could install a wiring kit to use the toad's own lights or use magnetic lights.
