diamondback (the vehicle engineer) did a pretty good job answering this question, so I'll let him do the talking.
Well, nobody really expects milk to pour out of the Milk tanker you can attach to the T-series, and even if it was suggested i doubt they'd add it, much like if people suggested functioning water. in short, it would be a great thing to add I think. new idea that barely any other games add in that concentrate on driving and physics, and you would finally have a semi realistic simulation of what it's like driving a pump full of 1000l of water.
Kemro has made 2 fire trucks, they don't have functioning water of course, but they look good: https://www.beamng.com/resources/fu...stom-lightbars-and-selectable-patterns.16101/ https://www.beamng.com/resources/fu...stom-lightbars-and-selectable-patterns.16213/
Literally there have been many threads suggesting this lol, also for water and fuel, and saying that the trailers should explode
