I've always wondered why do most RV's and conversions have a Black, Grey and Fresh tank. Is it some kind of standard? Or is it simply black water tanks are more expensive to empty... because I'd like to simplify my system as much as possible and run only 2 tanks, fresh and waste.. Is there a reason I should not or should do that. I also will have limited space for 3 tanks, which is why i'd prefer 2, as my bus has a side mounted fuel tank and AC components in the back, leaving me with limited space for 3 tanks, but 2 would fit comfortably. I wouldn't mind a slightly higher dump fee but if it's drastic I'll reconsider...