We have the 29KS and don't use FHU sites very often. We often add fresh water from a 5 gallon container on longer stays. We minimize waste water in the grey tank, and drain some when necessary (very seldom). I tell friends when you by an RV you not only have to become a plumber, but you also become the sanitary district. You have to budget your consumption and waste like you would your money. We all do it.
I think you wouldn't be satisfied with much larger tanks because it would change the layout and features. If you go to a large Class A you can get more capacity.