This is all about conservation, the black tank may last you for 11 days with 2 people with extreme conservation, it is also helpful there is a bathhouse at the campground where you can shower and use the restroom during the day saving the RV tanks for night time use only. For comparison on non-active conservation, my coach as an 80 gallon fresh water tank, 40 gallon black and 40 gray, though the bathroom sink does also feed to the black tank (I shaved daily at the bathroom sink, though I would turn off water while lathering, and had a slow trickle to wash off the blade). I recently took a week long solo trip where I did not actively conserve water, took a sub 3 minute shower each day (digital shower timer), and flushed each time the toilet was used. I made it the full 7 days, but 8 or 9 would have been pushing it.
