I used plastic containers with tops frozen into the ice so that food was frozen below the level of the ice but accessible from the top. Think NY steaks. It's no fun to chisel out the food.
Decide on which coolers will empty first and put in more food and less ice in these. On our trip after the food was out of the first two coolers we lifted a two inch slab of ice out of the cooler and put it on top of the other coolers.
On you last cooler use some dry ice to keep the ice from melting for the first few days. You'll have ice in this cooler for 27 days.
Remember if you put lettuce and other fragile produce in direct contact with the ice it will turn to slime.
Pre cooking is one option but adding canned and dry ingredients to frozen food will save a lot of room in the coolers. For example if you are making chili you only need to have meat in the cooler. Onions canned tomatoes act. will all come out of dry boxes.
Dry ice will freeze blocks of ice with water in between into a solid block. If you try to freeze too much water in a cooler all at once it won't freeze all the way through and if it did it would expand and wreck the cooler.
Why does Pluto walk on all fours, drink from a dog bowl, and get treated like...a dog, while Goofy drives a car, wears clothing, and speaks in English?