Pee in a bottle, or somewhere around the tank if you’re at a temporary stop. If you’re at an extended stop (an overnight, or just several hours), we’d go to about 50% security (usually someone on the TC’s .50 cal, and another on the loader’s -240) and following Army field procedures, make a temporary latrine.
Take a chunk of camo netting, a pole and spreader, and some tent stakes, and you can make a decent ad-hoc field latrine that gives a bare minimum of privacy to do your stuff. Mostly, no one wanted to look at someone else’s hairy ass making a poopy.
An MRE case, properly stuffed with dirt/sand, makes an acceptable field expedient toilet seat. A small packet of toilet paper came with every MRE, plus there’d always be a roll or two of regualr Army toilet paper (aka “tree bark”) in one of the sponson boxes.
MREs are great for slowing down bowel movements, but when you gotta go, you gotta go. This never happened to me, and I don’t think this ever happened to anyone I knew; unless they just never talked about it.
I can tell you any such conversation would have went, had it occured:
Soldier: "Hey Sarge. I gotta go.
Soldier: “I mean, I gotta go #2. Real bad. Right now!”
Soldier: “If I don’t go soon, I’m gonna shit myself!”
Me: :dubious: “If you shit yourself, you’re getting an immediate transfer to the Infantry. And by ‘immediate,’ I mean as long as it takes me to toss your poopy butt off the tank!”
Anyone (well, I guess almost anyone) can sort of train themselves to pee/poop at will, and soldiers get good at this. We could pretty much sleep at will, too.
Something any reasonably experienced soldier learns to always take advantage of when it comes along:
In Desert Shield, in the immediate lead up to the air campaign, my unit was moving towards the Iraqi border in stages. We came upon a kind of trading post at the corner of no-and-where, Saudi Arabia. The proprietor was a little leery of us at first (nothing like a company of tanks rolling up on you as a way of saying “Hello!”) Since we just about cleaned him out, he was weeping for joy and kissing us by the time we rolled off.
The first thing my driver grabbed was a brand new, unopened, pack of underwear. It seems he didn’t stock up on new underwear before deploying, and 3 months later, his stuff was wearing kinda thin.
My platoon sergeant got a couple dozen eggs, some cheese, onions, and peppers, and a little one-burner camp stove (I was in HQ platoon, so the SFC was in a HMMWV). Sergeant Whatshisname was a pretty damned good camp cook.
We had omelets for breakfast for a couple of days. Which was a very nice change from MREs 3 meals a day.
I found some garlic powder, red pepper, and hot sauce.