I nearly bought a used service bed a couple of weeks ago. I'm thinking now to go with a semi-flatbed that has fender arches and build out storage fore and aft of the rear tire openings, above and below the deck.
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Any idea how low the water pump can draw from?
Are you going to passivate it before use? No idea what that might cost, but it would eliminate any impinged steel or iron in the stainless steel, which could 'color' the water's flavor. I think that I would make the baffles in a zig-zag manner to add some lateral strength to the middle of the tank. Adding flanges to them would mean that you could rosette weld them from the top to tie them into both horizontal sheets.
You'd loose the structural support part of the tank, but I'd be much more inclined to have the tank fabricated from polyethylene. I know that there is a tank mfg in the greater Victorville/Hesperia area that can do this type of work.
Don't know of any QD's specific for water. Were I doing this, and I've contemplated a tank where the spare used to live under the bed, I'd just use AN/JIC fittings with a teflon or polyethylene lined hose. Probably more AN than JIC since they're anodized and will be less likely to corrode. Alternately mcmaster.com's "Instant Fittings" (aka "poke - n - hope") with a flexible PE tube might work well. My biggest concern for both of our tanks is freezing weather. "PEX" is probably also a reasonable option.