We have found when we aren't planning any travels for a bit to turn off the leisure battery using the red switch/knob in the electrical cabinet results in keeping some charge on the leisure vs. a full drain. Then the day before a trip we turn back on and plug into the garage electrical to charge back up.
We purchased our GC used, and the day we went to pick up the leisure was completely dead, and the dealer actually took it to the VW dealer up the road and had a new leisure installed, as they couldn't get the original to hold charge. We were very happy with this action as we had driven from Slovakia to Dusseldorf to pick up!