Ronco has a 5 gallon tank which fits that space on an EB really well. I put one on each side to provide 10+ gals of total holding tank capacity. I can get the tank model number if anyone is interested, just let me know.
With a little modification, they could be made to sit sealed flush up against the bottom of the floor and opened up as accessible storage from inside. I think it is a difficult spot to make it a drop down removable from outside storage area due to the debris and dirt kicked up by the tires.
Passenger side before crossover connection:
View from rear
View from front
Driver side with dump valve:
Large grey center box is my underfloor storage box accessible from inside, also a Ronco tank, 55 gallons capacity