I dont have your set up but it would seem to me that if the tanks are connected through a relatively small pipe than perhaps you filled quicker than the tanks can equalize. Meaning it worked until the tanks equalized and then became to low.
Or, if the connection between the tanks is on the sides and not bottom, than perhaps you had enough water at first, but after driving some spashed into the scound tank?
Also, if the tanks are level to each other, than they should drain at the same rate making added sensors pointless.
Silverado HD 2500 6.0 Crew Cab
2014 Roo 21ss