Towing an EMPTY tow dolly behind a Fifth Wheel east coast.

State police, or the state DMV. AFAIK, a tow dolly is a trailer, especially since it is attached to the tow vehicle by a simple hitch ball. Some states consider anything not attached at the factory as a trailer. Check with the DMV, but don't rely on that because IME, the DMV is woefully misinformed at times. But ya gotta start somewhere. :)