It's probably the same fridge I have in my '74 Argosy. There are 2 modes; GAS and ELECTRIC; In order for it to work on gas, you must select the gas mode and obviously have the propane turned on.
In the electric mode the fridge must be plugged in and the shore power must be on.
I've attached a photo of what I think the controls on your unit look like. In the electric mode, set the electric temp setting to at least 5, it will take a few hours to start cooling down.
You also need to make sure the fridge is level. Put a torpedo level on the bottom shelf of the freezer and adjust the trailer accordingly to get the fridge level. These old refers don't operate well when not level.
If you have not had the gas on in the trailer. I find it easier to light the cook top first. The burners on the cook top use a lot more gas and will fill the propane lines quicker. Even at that, the burner in the fridge is pretty small; it will take some time to purge the air from the line feeding the fridge.
If you can not get it lit (in gas mode) by following the text on the photo.
I can show you how to do it from outside.
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