Another thing worth noting is that refrigerators are pretty well insulated, and stay cold inside a long time as long as you don't open and close the door. Mysteriously, people don't tend to open and close the door a lot in a trailer while it's in motion, so if it starts out cold, it stays cold! Our strategy here is to plug our trailer in and load up the 'fridge night before we leave. After a day's travel the frozen stuff is still frozen and the milk and sodas are still cold.
Some people here have also suggested freezing water-filled pop bottles and stocking those in their 'fridge to keep it cold. It takes a lot
of heat to melt two liters of frozen water, much more than a 'fridge is likely to loose in a day of travel with the door shut.
So the simple man's way to keep a dual-fuel system cold while you're in the road is to stuff one or two liter-sized pop bottles of water in the freezer with your other freezer food the night before you leave. At the end of the day's travel you just plug in or turn the propane on and get the 'fridge/freezer cooling again and re-freeze your bottle(s), and you're ready for the next day's travels. No running your battery
down, no leaving the propane on.