I humbly suggest the question be restated differently. Like how much water do YOU want to carry and how often do YOU want to dump the grey tank? (If you're a pirate like me then leave off the dumping grey tank part ------sometimes).
Sometimes I think we don't fully develop "common sense" because it is either outlawed, or at the very least, highly regulated. I'm in no way suggesting that the origninal question is not a good one. It's just that a suitable answer depends on many variables and regulations that don't always follow common sense. If you have ever backpacked or "dispersed" camped, it is generally acceptable practice to bury human waste in a 6 inch deep hole at least 200 feet from any water source. Some readers may be thinking, "How outrageous! So much much crap in that 6-inch deep hole so close to the water!!! This is not responsible at all!!!!!!" Interestingly, the distance between my 1000 gallon house septic tank and my well is about half that far. Doesn't make sense but I didn't make the rule. If you ask me, there's an interesting correlation between hole/tank size and water available and ability to move the water.
As I equate places where the water table is so high as to prevent burying waste or draining grey tanks in a hole to be like a swamp and unsuitable for camping, I would think most people are safe to dispose of their grey tanks in state/national forest lands where they are camping unless otherwise posted against such practice. Here's a quote from a National Forest website: "When camping in remote areas without toilet facilities, people should move away from streams, and dig small holes to serve as toilets and then cover them well. Gray water should be dispersed on the ground in a wide area, away from streams, and away from the campsite area" (1). Also read the Suggested Gray Water Dumping Guidelines at the link at the bottom (2). Hey, if it's good for backpackers and tent campers it's good for me too! Just common sense if you ask me.
Depending on the state you may find yourself in, shower water may be classified differently from the kitchen sink water allowing the shower water to be drained pretty much anywhere but the kitchen sink water must be disposed of the same as the black tank at a dump station. This sort of simulates using the outside shower (or washing a car) to run off naturally without the guilt associated with the grey tank dumping on the ground. Yup, you read it right. So break out that hack saw and cut that shower drain pipe so you can install a shutoff before the grey tank and tee off a water hose. DON'T use that prestine blue or white drinking water hose though. Anyway, I again suggest we use common sense in the matter.
Since an RV is generally considered a temporary residence when at a location other than a campground, we are natually conserving more water because of the hassle of getting more water too often --- at least while boondocking
. So go ahead and take those Hollywood showers and don't worry about the amount of water commonly used. You'll run out of water soon anyway and then it won't matter. Just make sure you dump it out under where your exterior shower is. That's just good common sense.