I haven't done it because I dislike strings hanging down but it would be easy to do. Usually, with a double bulb RV fixture, there is a dividing strip of plastic between the two lenses. That strip may house the original switch and it may not. Regardless, you want to mount the pull switch in some part of the fixture housing. If you have the plastic center divider and the original switch is there, that's an ideal location for the new pull switch. Drill the correct sized hole in the fixture and mount the switch.
Your existing fixture and switch will have two wires into the switch. Remove or cut one of those wires. Splice the two cut ends or add another wire to the new switch so that your new pull switch is between the original switch and the removed/cut wire. Replace fixture into location, you're done.
