I did the same method, except with much less care. I just got back from Main Salmon. 8 hours of driving rigged to the river, 5 days on the river and a day of driving back (rigged). My cooler was exposed to the sun the whole time. On the river I did put wet towel on it and didn't let the kids get into the cooler. I came home, parked my trailer in the garage and my water jugs and beer is still frozen in the cooler.
Make sure you drain the excess water from the cooler each day, it seems to keep the cooler colder longer.
I did throw veggies in the cooler before the freeze and some of them went rotten from the freeze, so it's good to know. Also, any water/beer in the cooler will freeze completely. We had almost no drinking water the first day on the trip because all my water bottles were frozen solid in the cooler.