Can anyone suggest a way to stop the inside of the water tank from rusting too much.

Replace the rust pot with a plastic tank

Drain all the water out of the tank.

Fill it up again with antirust paint, and then drain it again. You will get every single spot, and it wont ever rust again.

you can use zink like they use on boats marine supply stores sell them we have ones that are like a half inch threaded plug replaceable zink rods

Use them and fix them when the rust have eaten a hole in the steelplate...And when it is no more steel to weld in do like the Indians ! Buy a new one ! ;-)

go aluminum or plastic if you replace if not keep patching,until you have to get a new one if its a putz and its a newer truck they may help you out with one, 2 of our trucks the tanks were cracking due to no shock absorbers under the tanks I dont know if this is the case with you but if its on a boom its something you may want to look at