On a 1977 RV refrigerator, it probably is two way, and could be three way. Inside on the front of the refrigerator there should be a selector to choose between the different power sources. That is where you start. Locate the inside controls and see what you have. If there is no selector at all, then it would appear that you have a propane only refrigerator, but that is very unlikely. While you are there, look to see what brand it is, and if you visit the website of the manufacturer, you can likely down-load directions or a manual. If you can't get the manual for one that old, you can do come tests.
First, since it is an older refrigerator, use a shop vacume to clean the area where you see that has been used for lighting. Be sure that you get it as clean as possible. Next, make sure that you have propane in the tanks and I would also use soapy water to check all connections for leaks. Once that is done and there are no leaks, you are ready to try and light it.
Open the little window where you light it and get one of the butane lighters for a gas grill, rather than a match. Look at the control knob on the valve as it may have positions of off, pilot, and run. Make sure that the ball valve is open. Set the knob to pilot and insert the lighted butane lighter into the hole. Now press the red button down and hold it, watching the inside for a flame. You should see it light and it may take a minute or two since there is probably air in the line. Once lighted, remove the lighter while holding the red button down. You will need to hold that button for a minute or two as there is a safety thermal couple that must heat up to make the unit operate. Once it will stay lighted, look for some type of run setting to change the control to, it is has one. There should also be a temperature control that can be adjusted.
Keep in mind that since your refrigerator is nearly 30 years old, if original, it may not work as they do have a limited lifetime. If it does not operate it would likely not be worth spending a great deal to repair it. New RV refrigerators have electric ingnters and automatic power selection to change from propane to electric power when available.