Salt will lower the freezing point, but I wouldn't expect salt to have an effect on temperature retention. Perhaps it has something to do with an earlier phase change from ice to water? Water is more dense than ice (at least the ice you are dealing with), saltwater can be colder than fresh water, and water completely surrounding your food will keep it colder longer than pockets of air and ice at the same temperature. But of course locking the food in solid pieces of ice with no air will keep it coldest.
The addition of salt to the gallon jug leaves less room for water, and the lower freezing point means that the temperature difference across the insulation is a greater span. I.E. keeping food below 40 degrees is about a 35 degree span to maintain, but keeping the frozen foods frozen needs to be around 15 degrees to be certain that all of it stays frozen, so the span is a 55 degree difference. That's my theory, anyway. I didn't expect the salt ice performance to be so poor.
I think the big gallon ice cubes are good as they should melt slowly, but their size and shape may make it difficult to pack tightly. You want to limit total surface area of your food and ice.
I have a pretty big cooler, so I can fit four gallon jugs (one at each corner) at about 32 pounds of ice and another 20 pounds or so of loose ice cubes; this will keep the food below 40 degrees for at least 4 days in 75-80 degree weather, out of the sun. I have a large deep freezer, so I can keep much more than that.
Have you considered clarified (ghee) butter? It last longer, might not need refrigeration, sautes well, tastes nutty, and you can make it yourself.
Considered it. Can't stand it. Won't even bother to ask my wife to try it.
