Unfortunately, you dog may be the problem. Even if you pick up all his/her droppings carefully, some are left behind. Mostly all species of female flies lay their eggs in decaying materials, that create warmth, such as decaying vegetation, and this includes very small parts of dog droppings.
If you notice more flies the longer past 7 days of staying on a site, this is probably the cause as the leftover droppings build up.
We experience the same thing in our travels. At about two weeks, the local fly populations rise.
If you have this problem only in one location, there is a possibility they are coming to you. House flies can and will fly up to a mile radius from their breeding and/or food source. (Two miles if there is a steady, prevailing wind.)
Happy camping.