Hi, I would second the above: the instructions that came with our Coleman Extreme said put the ice/freezer blocks on top of the food as cold air falls to the bottom. You can always put food in containers to stop it getting soggy as the ice melts. I put bubble wrap on the top of our food and then rested the lettuce etc on top so it didn't freeze but kept cool. Buy ice in blocks if you can or, freeze water in milk containers before you go as a good alternative for a weekend.
I put a plastic mesh grid on the bottom of the cooli box, same thing that bars put their glasses on. The cool blocks on top of food. Cold air is heavy and drops to the bottom and with the mesh grid you have a layer of cold air. Also stops things getting soggy. As has been mentioned freeze milk and carton of fruit juice.
The 'cold air sinks to the bottom' rule only works when there is space for air to circulate. If it is packed solid, then starting with everything frozen as much as possible, placing ice packed all around, and disturbing the contents as little as possible seems to do the trick.
I always put everything that will freeze into the freezer the day before I leave. Everything else goes in the fridge overnight including the cool bag. There is usually room for it in the fridge and it means everything starts the day colder.
I always put frozen milk at the bottom but the freezer blocks as near the top as I can. If there's anything squashy it goes on the very top of course.
Yes your quite right, cold air does sink. So what happens when it has has sunken to the bottom with cold air still sinking on top? It starts to rise again and 'well up', filling the whole coolbox with the same temperature of air. It would make no measurable difference if you source of cold was coming from the top, side or bottom as it is a sealed chamber. If you've ever had a play around with dry ice in a bucket for example you can clearly see what happens from the gasses that omit and how it 'wells'. The same happens in valleys and hollows in the winter and the temperature difference over a few hundred metres elevation is only sight.
Put your ice wherever is easiest, if you've got the ice/food ratio correct then your food will stay fresh for just as long.