Maybe this would work if you didn't hook the tank to the coolant lines directly, but instead bypassed the radiator and hooked a heat exchanger with a separate loop/pump to the tank:
That would allow the regular thermostat system on the truck to function as designed (you're just cooling the coolant loop with a liquid-cooled exchanger instead of the original air-cooled radiator.
You would be limited how far you could go before the tank water started to boil off though.