I think you mean "converter"- that changes shorepower (120v) to 12v. An inverter does just the opposite, and is unlikely to be present in a rig the age of yours.
That said:
The converter is only necessary for 12v operation when running off shorepower or generator. 12v power is otherwise supplied directly from the battery or tow vehicle.
I have no clue where a converter would be located in your rig- try following the black (hot) and white(ground) wires from the pigtail; or the incoming 120v line. They all should eventually connect to the converter IF your trailer has one. Not all that vintage do/did- some just had two separate "systems" and used 12v when off grid and 120v when on.
If you do locate a converter, it likely has a switch marked something like "batt/off/shore". Switch must be in the right position for whichever source of power you're trying to use.
Per lighting fridge with a busted lever:
Lighting can be done from the back of the fridge, too. Turn fridge gas control to "on"; go outside and open the access hatch; there's a little flap-door on the right of fridge back-bottom you flip open to see the burner. A red button about dead center bottom is the gas pilot feed; while holding a lit match to burner, depress and hold upward the red button. Gas will flow and burner will light. Continue to hold button for one minute, then relaease. Flame should stay on.