Hopefully the common and (relatively) low cost water container(s) work for your setup. If not, you might want to check Ronco Plastics water tanks, ranging from 3 gallon tanks to 380 gallon tanks:
Water / Waste Holding Tank - Ronco Plastics
Marine tanks are suitable for all marine plumbing, waste holding, and water supply needs. The tanks are rotationally molded of a seamless one piece, non-corrosive linear polyethylene and meet FDA regulations. Three female NPT (national pipe thread) fittings are provided free of charge...ronco-plastics.com
I saw one at Amazon, 21 gallon that measures at... 34" x 9" x 16", which will EASILY fit under the bed, and allow me to reduce the height of the uprights to 10", and still have LOTS of room for the 12V pump, fittings, plumbing etc...
They also have a longer at 39" 25 gallon tank that is sold singly instead of paired up with a 16 gallon... but I digress.
Now consider 1 gallon water daily for cooking and drinking, and 2 for washing up / bathing that leaves me with a capacity for 7 days. My intent has always been to have a between provisions stops at around 5 days, and of course fuel is going to need to be taken care of more often than that.
I think for my intended purpose, I can probably get by with using 2 of my AquaTainers in the back of the cab, or just riding along in the aisle in transit for now, their only purpose is to provide water source for showers and dishes after all, and a few of the grocery store 1 gallon bottled waters refilled when safe and often.
My long term goal as I have mentioned before, is to build a "foamie" pop up Project M type camper shell. WIth that in mind, I can easily go with as much as a 30 gallon fresh tank without gobbling up too much room but I may end up with more weight than I want to deal with in a half ton truck... I was trying to stay around 24 / 25 gallons fresh total for weight reasons...
This does leave me with the issue of where to store / haul the toilet. WIth the bed cut down now. My total height on the bed frame is going to be 14", my total height on the toilet is 17". I can grab a 2.6 gallon toilet but that would reduce my duration between dumps of the black tank, not something I want to accelerate.
So much to try to cram into such a small space... Ugh.