We boondock almost exclusively and never really worry about the grey water as you can drain it into a hole in the ground or a container to get rid of it.We also have a black tote(for poop) we use for long stays if we have the ATV trailer with us which we carry more water in 25-50 gallon barrels and larger 25 gallon propane tanks.And also,a porta-potti is never a bad idea to have around out in the sticks.
Holding tanks just never seem to have enough capacity for longer stays in the hills in most RV's.Tenters and hikers just dig a hole and bury there stuff out here atleast.