Carl's right--you're fighting something in the water. Your chlorine loss overnight should be less than 1 ppm. It will be more during the day, but shouldn't be nearly as much as you're reporting. You need to shock it and hold it at shock level as consistently as you can to kill off whatever is creating your chlorine demand. As far as vacation, your plan sounds good--but if that bleach-adding relative could come by a couple of times, maybe on day 2 and day 4 or 5? it would be even better.

Janet