First of all - Welcome to Forest River Forums! Hope we can help you.
You really do need to give us the make and model number of the fridge (e.g., Dometic DM2652).
As Bikendan said, your camper might be gas/elec but it might not considering its age. The newer Flagstaff Micro lites are gas/elec.
If it has a control panel that looks like this one ...
Than, the following applies...
To allow the fridge to choose between AC or Gas
- On/Off button is depressed to "on"
- Gas button needs to be pushed in.
- The "auto" light will light up and it will choose AC first and then gas if AC is unavailable.
If you wanted gas only
and not AC
- On/off button is depressed to "on"
- Gas button needs to be out.
- The "auto" light will not light up.
The gas button "out" effectively locks out the electric.