Not sure what era you are going for but I can tell you that currently, 3/4 ton trailer markings are placed on the back of the box in a similar manner as vehicle bumpers. I've seen some units put the markings on the front of the trailer, but that's probably a 1SG going over-kill. The code should be the same as the vehicle the trailer is normally assigned to, plus a "T" on the bumper number. For instance,
Truck:
20E 307E HQ-402
Trailer:
20E 307E HQ-402T
(This would be a truck and trailer in the S4 shop (supply) of the 307th Engineer Bn.)
I've seen water buffalo's get a "W" and generators get "G", I don't know if these are official from the regs, or something a company XO made-up to be different.
Hope that was of some help.