Tank wagons carry anywhere from 300 to 2500 gallons of water, and normally have a 50-1000GPM pump. So if you ask me for EM4's water script I would say make the GMC (and if you make it, the retro mack) a "light" tank wagon with 2000 gallons of water on it. And name the Kenworth bullnose, the standard Kenworth, and the oshkosh in as a "heavy" tank wagon with around 3000 gallons of water. That's just my idea.