Last year was my first season RVing with wife and 2 young boys. It takes some coaching with the kids so they understand why to use less water and how to do so. We had a 6 day boondock trip and I think 3 days was reasonable without pushing our luck.
That is with:
60 gal fresh water
35 gal black water
70 gal grey water
I planned in advance and new that half way through the trip we were going to pass through a town with an RV park. I had emailed them in advance and knew the price for filling my fresh water tank and dumping my other tanks. It worked out well.
I also learned we consumed about 1 gal of propane each day with us running the furnace at night (it got cold).