In my first bus conversion, an MC-7 Combo bus, there were two stainless steel holding tanks, each with a capacity of 250 gallons. Those two tanks were custom-built to fit into one bay in the belly of the bus. I could go a month before having to dump and fill them when I traveled alone. If my girlfriend was with me, it would last about three days.
Yes, I know, a gallon of water weighs 8.3 lbs., and additional weight in a bus conversion can be an issue. If both of my tanks were full at the same time, I would be carrying two tons of liquid in my bus. But everyone knows there is never a time that both tanks would be full at the same time. You fill the fresh water tank, and over time, it gets transferred to either one combination grey/black tank or two separate grey and black water tanks. Carrying one ton of water or sewer in a bus is quite a bit, but most buses can handle that much extra weight if your bus is not loaded with granite countertops or other heavy objects.
Another advantage of having large holding tanks is that you pay a fixed price per dump, not per gallon, at most dump stations. So for the $15 that some places are charging now, you would get more bang for your buck with larger tanks of course.
Bottom line, if space and money were not an issue, what size Fresh, Grey, and Black water holding tanks would you build into your next conversion?
