I have previously extolled the virtues of the LC Industries water cans, which are basically the same as the Canadian Scepter water cans, except in the USA. Hard HPDE plastic, and way beefier than the other cheapo kinds. No BPA, wide lids, 5 gallon size.
Anyway, I made mine into a dispenser by modifying the lid. I added a tire valve so I could blow air into it, and added a PVC dip tube and hose bibb so I could attach a normal garden hose. Put the cap on, blow in a few PSI, and out comes water. Not tons of pressure (I think you could blow the whole thing up with too much air; about 8 PSI was enough for me; the can would probably leak at over 10).
If you attach a continuous source of air, like your on-board compressor, it will of course pump indefinitely. I found I'd need to add air as it drew off the first couple of gallons. And you need some headspace to get the air to work; if you can is full to the top you have too little air to push.
The spout in these is 3/4", so 1/2" pipe or PVC fits snugly in and holds air with some teflon tape or glue. You can use a metal nipple, or the bibb, to self-tap into the soft plastic. Some 1/2" hose, like the braided vinyl type, is also 3/4 OD and fits snugly in the spout if you want just a flexible pour spout. Might drip a bit.
If your can is mounted higher than the tank in your van you could just use a couple of squirts of air to start a siphon; with the dip tube you could even leave the can in your rack.
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