LEOs do stop and measure. In SD length is now 75'. used to be 80'. Multiple tow allowed, ball and ball. Most states allowing multiple tow require the first trailer to be a 5th or goose, longer than the second trailer. Most states regulate length and towing multiples to interstates or near them on state # highways.
SD used to allow 80', so once upon a time when returning home from fishing with our 32' tt and 20' IO boat an LEO stopped us on I 90 for a safety check. He measured us at 80' 4". made me lower the IO drive. Congratulated us on sway controls and braking for both.
Another time in 2008 we were going to Gulf Shores Al., through Ark., wboth states which prohibit multiples we were towing our 35' 5th with a M/C trailer total near 65'. A sheriff at a fuel stop in Jonesburg, Ark. commented its illegal here, legal where you live, you have gotten it this far. As long as you can handle it without trouble, we will let you pass. He did not say there is reciprocity between states, rather he intimated tolerance for our visit, severe penalty if we had a problem.
I am not advocating breaking a states law., rather pointing out that with proper equipment that is legal in your home state that in others where it may not be LEOs may let you pass by. You may risk severe penalty.
An interesting anomaly is N & S Carolina about multiples. I discovered in a thread I posted last Oct. inquiring about multiples in those states. Its legal if you are in nascar motorsports. Not for rvers. Now theres a successful political lobby for you!