My wife, son, and I can easily go 2 days with the fresh water supply including conservative showers. If we had to we could stretch it to 3, maybe 4 days with really strict water usage.
Regarding WH draining... I empty mine after every trip. When we had our popup, I once left water in the WH for about 5 weeks after a trip. When I did drain it, the water had a rotten egg smell so from then on I just made it part of the post trip routine to drain and flush the WH. The water sources of most of our camping destinations are relatively hard so I don't want that water just sitting in any of my tanks. I have seen the effects on my home's plumbing including massive mineral buildup in old WHs I have replaced. If I was able to camp as frequently as I would like then I probably would not worry about draining it as much, but if it is going to be more than 2 weeks I take the time to drain it. Besides it only takes 5 minutes to drain and flush it.
Michael, Cristina, Anthony, Cocoa Puff, Cookie, Blossom, Tosca, & Heidi
2012 Rockwood Roo 21SS