The theory is sound. As mentioned above, several rigs came this way. Our '91 350LE MH is so equipped.
The only problem I see is perhaps in the SeeLevel system.
I expect the signal from the 3/4 or full sensor on the tank is not a simple 12 volts.
It is probably some very low level voltage, more like microvolts, or to be more technical "logic" level signal.
The rub will be converting this logic signal to something useful and able to switch your valve on and off.
Transistors, resistors, and diodes kind of stuff.
Even if the sensors were a full 12 volt
simple on/off signal, one would need a relay to turn off the valve after the manual turn on is switch activated.
A sperate level sensor for pump turn off, not part of the SeeLevel system might be easier to deal with.
Certainly not impossible, but a some knowledge and a trip to a well stocked Radioshack store would be required.