OK, I reviewed the manual and the electrical
diagram. The 12 volt switch next to the 120 and gas knob does control the 12 volt heater (IDK if its red). I know you did not do this on purpose... the manual says:
NOTE: NEVER OPERATE THE REFRIGERATOR ON
MORE THAN ONE ENERGY SOURCE AT A TIME.
But of course stuff happens, so it seems like there would be or should be a relay or something to lock out the electric heaters when propane is being used. However looking at the electrical
diagram and judging by their warning, it appears that there is not. Therefore I think the propane flame and 12 volt heater were both on at the same time. So yes, that will drain you battery fast if not on shore power. But my bigger concern would be overheating the unit. Not that I know it would be damaged but the warning is in the manual so I assume its possible, although it might take some time.
Dometic has good customer support so you could call them for the official answer. But if its under warranty you might want to test it on gas and 12 VDC first (on tug power rather than battery). They could probable see evidence of overheating from both being on at the same time (if that is a danger) but it might not be conclusive so they might fix it under warranty if its not working.
If it does appear to be working OK still then just remember not to use two heating sources at the same time going forward. Maybe some tape on the switch, or a cover of some sort. Its a bad part of the design IMHO.