I see that the output voltage of the converter is 13.8v. This is suitable for "float charging" a 12v battery. Once the 12v battery is charged, the converter should draw very little current from the high voltage input. (You could always check this yourself with an ammeter)
On my EV the DC-DC converter is permanently connected on both high voltage and low voltage sides. Even if left unused for a week, there does not seem to be much drain on the high voltage pack.
Fitting the relay seems to me to be a lot of fuss for little gain.
But if you are really keen to disconnect the converter when the vehicle is not in use, here is another thought. Will you be installing an isolating switch to disconnect the traction pack from the motor controller when the vehicle is not in use? (this is a useful safety feature, and can also serve as a panic emergency disconnect switch). If so, just connect the DC-DC converter to the far side of the isolating switch.
A nice big red button panic disconnect swith can be obtained from http://www.albrightinternational.com/la ... eld=&ID=63