Wabash Mfg (as far as I know its a different company than the dryvan manufacturer) up in Westlock, Alberta makes some nice tanks. Brandt bought out Camex in Nisku, Alberta but they might still manufacture tanks and such.
S/S to maintain the potability? Check out any milk tanks you see going down the road and check those manufacturers too; not everyone makes sanitary units and/or there could be other regional makers making what you need. Boston makes a lot of aluminum fuel delivery units with an extra false rear head hiding a hose reel that may not fly for potable with some people and agencies; I don't know if they do stainless. Fiber reinforced plastic [FRP] would have a good number of choices if it was permitted and would be lighter.
Why? you using for potable water? the two sitting next door used for GM are steel tanks with a coating.
“As far as I can tell…” you’re the business. It’s whatever you want or your customers want. Find out what you need first! and Google is your friend here. No offense to my fellow drivers, but I wouldn’t ask any of you where to buy a tank if I needed a tank!
The one I'm thinking of is already on a truck. I called the guy and he won't split them up. Nice tank, about 3900 gallons , on a late 70s 3 axel Freightliner cabover with a 400 BC and a 13 speed. He went all through the truck this past winter. I've worked a couple of fires where this truck was and it does a great job. Big Deutz pony motor with what I think is a Roper pump, all stainless piping. It's carded for CalFire and USFS as a potable water carrier so it has a good history. I think he's been going to fires for about ten years that I know of. If you're serious about hauling water I'll ask him what he wants for it but I know it won't be cheap.