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It should have batteries (or at least one) somewhere. For 12 volt items to work they really need a battery in the system along with the converter. Unless the unit has a residential style fridge it should run on gas when you switch off power. It will still need the 12 volts from the battery to power the control board.
Hopefully someone on here has a similar model and can clue you in as to where the battery is. On most fifth wheels it is in one of the front storage compartments, typically the one at the very front.