Try this....
Disconnect the alternator from the battery live feed, you might have a dodgy voltage regulator. The alternator is wired permanetly live. Alternatively (no pun intended) the problem could be with the auxilliary power supply socket; however dog nosing at a distance is difficult.
Put a volt meter across the battery with both + and - leads disconnected note the reading.
Reconnect the battery leads and repeat the exercise, note the reading, you should get a small difference to allow for current consumers such as clock an any electronic alarm devices (if fitted).
With the volt meter still connected across the battery terminals turn on the ignition, note reading, turn on lights if not conrolled by ign/sw note reading, repeat operating different consumers until you have voltage readings for all and then with everything turned off but with volt meter still across the battery you should from the noted readings be able to deduce the fault...perhaps.
"Now my dear Watson,
Sherlock will call this deductive process the voltage drop test. But as Morriarty nearly caught me out on more than one occassion I will need to go in disguise and check that the starter main feed wire has not be sabotaged by chaifing against the frame or some such!"
With that he leaned back in his chair and drew in a large amount of the opiate and began to play with his fiddle.
In all seriousness I hope you can find what you dealer should have found in about half an hour of reasoned and (in their case ) experienced diagnosis
