FreeLanceing,
I can't lookup every state but you live in Michigan and this is the: Michigan Vehicle Code as last updated on 7/1/2014.
Below is some of the laws in the 383 PDF pages. Weeks of reading if you so desire. Chapter 257 is the Vehicle Code.
257.224 License on all vehicles clearly visible from 100' during daylight hours and be a minimum of 12" above the ground. If operated during non-daylight hours, the license plate must be adequately illuminated and visible.
257.686 Pickup camper must have registration plate clearly visible.
Maximum pickup towed vehicle combination length is 75'.
257.698 No decorative fender lights etc, or any kind of lamp, reflector, flashing, oscillating, or rotating lights permitted.
This is the gist of the laws for pickup camper license displaying.
If I were you, I'd move your license plate to the rear of your TC. Why, because when I went down to the Michigan State Police Post here to have my rear 24" wide porch sticking out 25" behind the Lance TC's rear bumper inspected for being legal, I was told I had the move the license plate from the rear wall of the TC to the rear of the porch and illuminate it for night time driving, so I did. The Trooper (Motor Carrier Division) stated that the entire license plate must be clearly visible from any angle behind at all times. I made the 2' X 8' porch purposely for our trip to Alaska in 2002. This inspection was in April 2002.
Traveling to other states that may have even stricter rules will keep you from being pulled over and hassled or fined.
www.legislature.mi.gov
I'm now crosseyed and buggieyed after only reading about half of it as so much is burried and defined in the different sections!