it's a while since your post, so I hope you've sorted things out.
I had a similar problem with our Burstner (A532-2). The symptoms were no 12 volt supply in the back and (more annoyingly) no radio in the cab, even with the engine running and the mains cable plugged in.
I tested all the fuses, changed the fuses then took it to the dealer who sent me to Fiat, who sent me back to the dealer. After endless head scratching and testing, one of the mechanics asked if my leisure battery alarm had gone off. It had (while the van was parked up) but I'd hooked up to the mains and didn't think anymore of it.
Apparently, Burstner fit a low-voltage trip switch to stop the leisure battery draining completely. This cuts all the power to the 12 volt circuit when the battery goes below 11 volts or something. The only way to reset it is to turn the 12 volt switch on and off (the one on the control panel in the living area). Doh!
The cab radio on our van is wired into the leisure battery circuit. The light-up facia on the radio draws a constant feed, meaning that it gradually drains the leisure battery. Nowadays, I take the facia off and stow it in a cubby if the van is parked up. Haven't been affected since.
There's nothing in our Burstner manual about this and the mechanic hadn't seen it fitted before....