[Wombii]

Looks like a good plan. Looking forward to seeing how you'll fit it in the truck. From my tests I'd say there will be a surprisingly good flow from gravity alone for the dump chute, especially if the tank is tall.
I hope you'll have good tight plugs for the unused suction ports, as the pump will prefer to suck even a tiny bit of air instead of the tank water.
If you're only using gravity, you might be able to make a simple valve by bending a thin walled silicone tube with a servo. I accidentally bought a lot of silicone tubing that kinked badly when bending. It's used for fish tackle, apparently. If the dump chute is articulated or telescoping, you could maybe solve both with a single servo.

I don't know what type of pump you'll be using.
The real truck probably uses a centrifugal pump. Scale sized ones are commonly found as aquarium or garden fountain pumps. Single direction, very high volume but lower pressure at scale size. Needs to be below water level unless primed. (If you can fit a small one for the dump chute, it might do the job, but I don't know if it will leak air or water through when off).
I went with a diaphragm pump used in cheap high pressure washers or camper / boat kitchen sinks. Single direction, Large size, high pressure, good suction height, pulses the water flow a bit. Also very sensitive to a stray dog hair blocking the internal check valves, from experience.
Gear pumps are bidirectional and very high pressure, lower flow and good suction height. Sensitive to objects like dirt and sand. Scale sized ones are commonly found as r/c hydraulic pumps and r/c fuel fillers. I originally bought a nitro fuel filler pump, but wasn't completely happy with the flow rate. Two of them in parallel would have worked great though. Even though my gear pump had much lower flowrate when unrestricted, it could match the diaphragm pump when the nozzle was very restricted due to the extra pressure.
I think all types might be used as windscreen washer pumps?