Cant say I have ever seen that.
Only trailers I have ever seen a SMV sign on are our wood hauling trailers we tow behind the tractors. Which top out at 23MPH if behind the 8n
I have a smv sign on my tractor but was told to remove it from the tractor when hauling it on a trailer, then it is not a slow moving vehicle.
here in Maine speed has nothing to do with it. SMV emblem is required on farm trailers and equipment regardless of speed, they are registration and inspection exempt. This only applies to farm equipment, no personal use. Of course farm plates on the tow vehicle helps if questions arrise.