Not sure if it's relevant or not but, here goes.
I drive an IVECO Strallis artic. The current one is my third and they all had an automatic battery isolator. When this operates, it cuts feed to all but the tachograph and fridge.
There is, on the dash, an override switch. This has ON above a battery pictogram. Operation is by depressing the upper portion (as in turning the isolation off) to energise everything again.
If you have a similar thing fitted, I wonder if it may be faulty.