Look at the control panel on your fridge. If it has a button to select the mode of operation, AUTO or GAS, set it to gas. But, before you do, you might want to turn on a stove burner for a moment or two to clear any air from the lines. Both the water heater and the fridge are sensitive to air in the lines and will not light until it's cleared. Lighting a stove burner is the easiest way.
If you're on shore power, the fridge will normally use the 120V supply automatically. Setting the selector to AUTO will allow the fridge to automatically switch to GAS in the event the 120V power is turned off, providing your battery is fully charged.
This is based on how my DM2862 fridge works.