My jeeps are not pretty. So I have no problem throwing garbage in the back of them.
Regarding spill kits, I don't carry one.
I do carry a one gallon zippy bag, in case I need to drain something. But my normal disposition is that oil came from the earth, and can return there forthwith, at least around here. When I was in Moab, on the slickrock, it was more evident that an oil spill there would leave a lasting eyesore, and I'd be more inclined to carry spill absorbant. But in a pinch, I'd just throw my coat under it, to absorb the spill, then throw the oilsoaked coat in the back of the rig. You can also just empty out the icechest and use it for a bucket.
Regarding stuff I do carry; TP for wiping my ass. And no, I do NOT pack my **** out, at least not around here.
As a general rule, I pack out far more garbage than I ever have generated while wheeling. I blame the snowmobilers. Cause a hunter or hiker isn't going to carry a 12pack of budlight in a backpack.