while (Serial.available() != 0 )
TankVal[i] = Serial.read();
Suppose you read two characters.
LeftTankReadVal = 0;
LeftTankReadVal = LeftTankReadVal + TankVal;
RightTankReadVal = 0;
RightTankReadVal = RightTankReadVal + TankVal;
Now, why you feel it necessary to set the value to 0 and then add a new value to it escapes me, but you did not read two ints from the serial port. You read two characters.
Why are you delaying BETWEEN moving one side and moving the other side?