I travel with the wife and two daughters who avoid the campground restrooms. I find that the black water fills up just as fast as the grey. The black fills up even faster when we have guests such as during a day at the beach park when 10 or 12 are stopping by to visit.
With 40 gal black, 40 gal grey and 100 gal fresh on a 35 foot 20,000 lb chassis that adds up to 1,440 lbs so any more would eat into cargo or passenger capacity. On my old coach without slides the holding tank capacity was much larger due to the lack of all the hardware/weight required to accomodate the slides.
There are tradeoffs in fuel economy, chassis capacity, physical space and cost. It is a balancing act and the laws of physics along with the family budget all apply.
Target market also comes into play, who does the manufacturer think will buy the coach, a young family, older retired couple, dry campers, trade show hoppers etc. Will they use the shower and commode in the rig or use the campground facilities and eat out most often.
While I would like larger drainage tanks both black and grey I know that I do not have the space or weight capacity on my F53 to carry them nor does my budget or parking situation allow for the purchase of a larger coach with greater weight carrying capacity.
2001 Winnebago Adventurer WFG35U