After that bang you suffered, I wouldn't be surprised if a wire somewhere has become trapped and damaged. Check too that the additional wiring and switching for the second battery hasn't been damaged.
Before you start getting into measuring the current drain, I'd be doing a careful visual check of all the electrics, especially those forward of the bulkhead.
I presume that you can't get into the back because the battery is too flat to operate the central locking.
Incidentally, what was the result with the insurance?