If the car is in a secure location and an outlet is nearby, set the charge percent limiter to 60% and leave it plugged in on the trickle charger for the duration. If you can do that there's nothing else to change and it's the safest bet, IMO.
If you can't do that then you can either charge it up to perhaps 80-90% and leave it as-is, or charge to 70-80% and disconnect the 12V battery. My only experience shows ~2% loss in 5 weeks when left with the 12V battery still connected, and that was in ideal temperatures.