Suggest working this from the top down.
What time period are you portraying?
Are you going to portray Army, Air Force?
What type of unit?
What unit?
If the supply truck for an infantry or armor company, most of these would be the company letter then a 4. A4, B4, C4
Below are some bumper number examples.
Remember what is on the front passenger side bumper is on the rear drivers side bumperette. What is on the front drivers side bumper is stenciled on the passenger side bumperette.
Deuce and 5 ton rear bumperettes would have the division over top of the battalion. (Note that some units stenciled the numbers on tailgates, this is somewhat rare. Note 2 is the 29th Infantry Division used a wide oval in sand with the bumper numbers painted in it.
Bumper numbers were normally sand background with black letters. On rare occasions I've see other colors including black background with white numbers.
Covered this many times on here before but there was a standard but the person painting them on was some private that had to do it as an extra duty for committing some infraction. I've seen front bumper numbers different than the rear. I've seen them reversed. I've seen them so sloppily applied folks got to redo it. I've seen them painted over with spray paint in combat. Although you can screw this up, you probably can't screw it up as bad as a Soldier.