It all comes down to Ohm's Law. V= I x R
Power = V x I ---> I^2 x R or V^2/R
So, half the voltage give 1/4 the watts. 1/3 the voltage would give 1/9 the watts, etc. If you put your elements in parallel, the resistance (R) is cut in half and would double the total power. So, at 120v, you cut the power to 1/4 and would double this if put in parallel to give 1/2 the rated power (total..each element still would be 1125W).
As you said, on 120v, your elements should last forever (probably until the tank falls apart). It is much like using 130v rated lightbulbs lasting much longer than the normal bulbs, since they are being ran at below design voltage.