It's really up to you as far as what sorts of markings you want to use. As far as official markings, I've seen trucks with U.S. Army and For Official Use Only underneath.
I've seen others with chevrons painted on the doors, mostly from Desert Storm, though a few woodland or plain green ones have had the chevron painted in sand color on them.
Sometimes units will stencil a chalk number on the side of the vehicle or something to identify quickly that it's part of a specific battalion, company or platoon. (This is where the playing card suits come in--spades for Company A, Diamonds for B and so on..)
Folks in individual units will often stencil their unit patch--like 1st Cavalry Division etc.,--or their branch insignia, such as the crossed pistols for MPs or the crossed muskets for infantry.
Mechanics will often stencil crossed wrenches on their trucks.
Every unit I served with we always stenciled the name of the vehicle somewhere on the vehicle as well as our own Combat Camera insignia.
Really, in the end, it's up to what you want or think looks good. There are so many unit devised or Soldier devised stencils out that that you'll never find in a TM.
Personally, I always loved seeing the ways in which Soldiers personalize their vehicles with stencils.