I hauled my golfcart from AZ to MI on a trailer I put together from a Harbor Freight kit. I watched it on the camera I installed in the back window of my fiver. The movement up and down at the hitch was like 1-3 ft, because of the overhang on the trailer.
No, I never made it down to Florida, but have been through North Carolina several times, West Virginia, and Virginia and never been stopped or even looked at twice. I always figured that if worse came to worst and I got stopped, it would cost me a bit of time and money, and a little inconvenience. It just wasn't worth worrying about as far as I was concerned.
Personally I think that, generally speaking, the law doesn't pay much attention to the swivel wheel type trailers. I think they would be more likely to notice a regular boat-type trailer first. It probably would depend on what kind of day a particular LEO is having.
OTOH if you're talking about a 12' trailer, that may get noticed very quickly because of the size. In this case, IMO, smaller would be better. Mine was 6' from the end of the 5th wheel to the end of the Smart Trailer, hitch and all, so the swing wasn't too bad, but you do have to be aware of it.
I think the 12' size was taken from the earlier post when i said that the overhang on my rig from rear tire to bumper was 12',i don't plan on making the swivel trailer more than 6' long by 8' wide.
In that case I think you would be golden.
But there are no guarantees you realize, you could be stopped and ticketed your first time through a state that doesn't allow double towing. Personally I don't think so, but always be aware that it could happen.
A company out of Detroit called Double Hitch Clicky ThingyMakes the swivel wheel leg and the other parts like wheels,hubs and such i have already bought from Northern ToolOther Clicky Thingy,after researching this to death i'm confident that this is the way to go.