Consider using some of those cooler/ice additives. I read about some stuff on Mountainbuzz, and tried it out on a 7-day Main Salmon trip in late July. Temps every day in the high 90s and blazing sun - it was downright furnace-like. We had a 110 qt. Yeti with several square-sided 1-gallon juice jugs with water plus the cooler gel in them. Two on each side and two in the middle. Those things were still 30% frozen, and very cold by day 7.
We used this stuff:
Amazon.com : KoolerGel The Ice Extender By TBK Industries LLC : Cooler Accessories : Sports & Outdoors
Food stayed cold the whole trip with no water in the cooler.
A few more tips we employed:
-Put wet towel on the cooler if it is in the direct sun
-Use cardboard or newspaper in the cooler in any dead space