I would test fire it before the rally. When you switch from electric to gas the electric portion of the heater will be off. Based on the vintage of your unit you will have a sparker of some type. These are notorious for not working or being broke. If that is the case then You will need your other half to help, if the refer is like my old Dometic was I can offer a blow by blow of the ignition process from the rear of the refer.
Open access door, if it opens up, if possible remove it, if removal is not possible then you will need an extra set of hands (helper)
There should be a square metal cover over the bottom of the flue, you will see the gas piping going into the bottom. This cover needs to be removed. Note: even if your sparker is working I would pull this cover and vac out the accumulated rust that has fallen on the burner This will assure you that the burner will stay lit and you will not have cooling issues at the rally.
With the inside knob on gas and the gas thermostat set on max (this is my way, it may not be the manual's way) find the white nylon lever on the back of the refer to the left of the burner. Push and hold up on this lever. It is the safety valve. Light the burner while holding the lever up, once lit continue to hold up on the lever for a minimum of 20 seconds. Once you let off the burner should stay lit. If not try again, but hold up longer.
If the sparker works, this can be confirmed buy a helper activating it after you have cleaned out t the rust, then the directions in the refer can be followed. There should be a sight window that will let you know the burner is lit, but sometimes it is difficult to see the blue flame. I prefer to see it with my own eyes.
Once lit I would check to be sure it is burning a nice blue flame and that it stays lit/cooling overnight.
Once the burner is lit and staying lit you can reinstall the sheet metal cover. I know that stephroberts had a 9 volt powered started added to her caravel
that had a real old dometic refer so she would not have to deal with this issue, as well as protecting her fridge from traveling blow out.