Are you talking about bumper numbers or registration numbers? I've got some pictures from an NTC rotation circa 2011 with cav bumper numbers if you're interested. The numbers on the driver's side are the company/troop name and platoon/truck ID. So for that first image, it's B troop, 92. 92 is a little weird because it was our troop's fister truck. The HQ platoon and mortars used it. But you can see better examples behind it and in the second picture. B 15 for example. B troop, 1st platoon, 5 truck. On the Truck Commander's side you've got the unit identifier. The 1 and triangle is for the 1st Armored Division. 1-13 stands for 1st Squadron, 13th Cavalry Regiment.
http://imgur.com/3ACUw8T
http://imgur.com/9DnglbR
As for using bumper numbers and unit numbers, it might be a good idea to use an inactive unit just to head off any potential butthurt. Hope this helps!