Military HMMWVs use 24V and 100A or 200A alternators. Assuming you could tap half of that, you have 1200 to 2400 KW. So that's theoretically 10 to 20 gallons per hour. M149 Water Buffalo is 400 gallons. It'd take 20 hours to fill, which isn't ideal, but for stationary ops, 20 gallons drainage per hour isn't atypical. Fill at beginning of op, top off when possible. Hit a water point if you drop too fast or need a quick refill.
Be very handy for lengthy field ops, FOBs, disaster management, etc.
Hell, I would have loved something like that. Often, we had to have baby wipe showers instead of field showers (pallet plus water bag) because we couldn't send the HMMWV or LMTV back to a water point. Under most exercise and damn near all hostile fire zone rules, you can't just send one vehicle anywhere. You have to travel in at a minimum a two or three vehicle convoy. I'm not saying it'd necessarily be fuel neutral, but the overall fuel cost wouldn't be as bad as first glance because you'd need less supply convoy movement.