Well let me correct a few facts.
First, as I said, the Promaster has room for bigger tanks than Winnebago has chosen. So they have made some design choices - water tank inside - which some other RV manufacturers do as well. If they had used a smaller fridge, and moved the water heater, then they could have had a bigger water tank (I'd choose a bigger fridge over extra water capacity any day). I get the idea of wanting all the fresh water and piping inside the vehicle. If the waste tanks are sized by some formula in relation to the fresh water capacity, or if they just used cheaper standard sizes, I don't know.
What I do know is that on b-vans, the black tanks are all around 10-12 gallons, which should be good for a week, I think. One of the "quirky" work-arounds, I suppose you are referring to the grey water pump-out. Well, if I was to choose having a macerator on the black tank (which many B's have) or a pump on the grey, I'll take the pump on the grey any day. Looking under the van, I can see that the grey tank could be 10 gallons bigger if needed, not sure why they chose only 14 gallons.
Thirdly, lets get the math right. On the CS Adventurous, the water capacity is 33 gallons, including the hot water heater of 3 gallons. To get apples-apples, then you add 4 gallons to the travato's 22 gallon tank for 26 gallons water.
You also made a math error:
So CS= 33 water+ 10 black +21 grey = 64 gallons.
Travato= 26 water +11 black + 14 grey= 51 gallons.
So a difference of 13 gallons. So is that "significant"? I'd be most concerned with the differnce in fresh water (7 gallons) and poo tank (-1 gallon). You can always dump grey on the ground. With the smaller black, I might have 1 less day out in the CS than the Travato.
In my "cheap" van I can store four 7 gallon jugs in my massive storage area including all my other gear. Can't do that in a CS.
Also, there is price. Roadtrek CS trades for around $100k to $110k. The Winnebago's are trading for around $70k. So you are paying a hefty premium to carry 13 more gallons of capacity. Not to say you aren't getting more van with the CS, but if you are focusing on tankage, that's what it is.
I am certain that other manufacturers, especially Sportsmobile, will make design choices differently than Winnebago that will yield better tank sizes. So what I'm saying, then, is that the tank sizes are a limitation of the RV designer, and not the chassis itself, as you are purporting.