I'm going to stick to my guns here. But only to restate the alternative view: there are very few advantages to a built-in water tank (compared to individual loose containers), unless you have under-chassis space. Otherwise, you need pretty much the same volume of internal storage space, no matter what solution you choose. (But containers can be moved around, optimising what space you do have).
A built-in tank sounds like a good idea, (and is perhaps a good idea if you have good water all the time), but you'll curse it when you unwittingly top up with tainted water from a service station, or suddenly see the dregs from the bottom of a well swirling up through your filler hose.
None of these things are make-or-break. But I'd choose to spend less money on what I think is overall a better solution, even with only two axles
If you do go the Nato plastic can route, it's also worth getting a through-the-lid outlet hose, to a 12V pump. The pump and tap can be permanent fittings somewhere convenient, and the outlet hose screwed onto whichever can is appropriate. When the can's empty, just transfer the hose to the next can. A push-on hose from the tap to a filter is handy too, for filling drinking water bottles.
Be aware with filters that they vary in quality, the best ones removing pretty much all suspended particles and the flora and fauna that might otherwise give you a very comprehensive guided tour of the local toilet facilities (or worse).
What they won't do reliably is to remove dissolved impurities such as poisons, salts, foul tastes, heavy metals, etc. There is no good way of doing that, short of distillation. Since most of the likely poisons and metals have a cumulative effect, it's good practice not to drink water for a long time from the same (untrusted) source.
You will almost certainly be safe drinking bottled water, though research done a few years ago in Nairobi, Kenya, showed that all but 2 brands of bottled water sold, turned out to be Nairobi municipal tap water, with no additional treatment! The same thing is doubtless true in many places (though perhaps not on the same scale!), but even so, you are unlikely to get anything worse than a bad bout of Delhi-belly from most city water.
Regarding quantities of water to carry, I have no idea what your dogs drink, but figure on at least 4 litres per day of drinking water for yourself, and maybe the same for the dogs. Washing and dishwashing
can be skimped on when you are short, but for normal use, I'd say another 5 - 8 litres. Call it 20 litres per day, and you'll be comfortable but not prodigal. Laundry and a more comprehensive shower can be done when water is plentiful. You can count on being able to get water of
some quality on most days - but you need to be able to go 3 days or so without, in some places. And you need a reserve for the engine, "lending" to other people, getting clean after maintenance or mud, etc. I'd say 80L in total would be sensible and ample, unless I got the dogs wrong. Always keep the last 20L or so for drinking only, if you find yourself down on stocks.
Rgds
M...