Ok, I'm going to assume that it actually does loose charge and not just stop working. 1st thing that comes to mind is it is venting through the pressure relief for some reason. That should leave evidence of an oily residue near the relief valve. These can be on the compressor or receiver. Causes would be the fan not turning on for low airflow over the condenser.
A loss of that magnitude should be easily picked up by a "sniffer". If you don't have one, it may be time to buy one.
You didn't say how long you left the N2 on the system, but I'm guessing nowhere near a week. N2 pressure can be effected by temp, same as any gas. Put it in the cool morning and check later in the heat of the day, and you might not detect any loss but some has escaped.
I assuming a day cab (most RD;s are) but if it has a sleeper don't over look the bunk unit and piping to and from it.
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