pnichols,
Our floor is lower than most given that Phoenix Cruisers have just a 1.5" step into the house from the front cab area. Also Phoenix Cruisers are less wide, just 93" outside width. It's tighter underneath than your typical class B+/C rig.
I recently was under our PC this summer installing
THIS back-up waste hose storage tube. We have a Sani-con macerator system, but I wanted to carry a Rhino kit in-case of a mechanical failure during a trip.
While underneath, I studied how Phoenix installed the waste tanks. They are hung under the rig in such a manner that seems designed to fit the dimensions of the E-series frame. It is an impressive hanging system. It seems the only way to increase the volume of waste capacity is to expand the tank downward toward the street.
Our 42 gallon fresh water capacity could be increased via a custom made tank specific to it's compartment, tweaking every available inch. Without calculating the extra volume, a custom made fresh water tank might gain 2 or 3 additional gallons of fresh water, maybe even more without other modifications to the rig. If I did some carpentry work, I could gain more than my two waste tanks can handle which is 58 gallons combined if filled to their tippy-tops.
With that said, I am sure every rig including that 19 footer would benefit from tweaking, but it is not financially practical to have custom made tanks for every RV model. Adding that their models seem to come and go with the wind, unique tanks per model would be costly to RV manufacturing.
Still, I agree with your point in that RV manufactures could do better with space utilization without additional cost. They just need to design smarter, maybe utilizing modular techniques for areas of their line-up that are consistent. The RV industry is generally a bit sloppy in that regards. Some manufactures much worse than others.
You have some interesting ideas on available space utilization. Some RV brands could easily do something with that space. If not for waste or fresh water, then possibly a secondary battery compartment for sealed AGM batteries for serious boondockers. I wonder if such a bolt-on battery compartment is available that could be mounted in the space you mentioned.
I wonder what a custom size fresh water tank would cost me.