Somebody made a commercial version of this once - a dolly/tag axle thing to pull a 5th wheel with a car/SUV. Which was different from the "Shadow" 5th wheel that actually hitched to the middle of a car roof!
There's a few different manufacturers out there. Here's one of them.
https://www.mortonsonthemove.com/what-is-a-5th-wheel-dolly/
