How much water I carry depends on the trip and destination.. Normally I try to keep it down to just a few (10) gallons, enough for "emergencies" of the personal variaty (prevention of true emergency if possible). however on my big trip I generally top off at the start cause it's 800 miles before I get to another hydrant. And the campground I'm in right now is working mightely to fix their water issues (Boil advisory) so I arrived with tank full.. Looks like I'll be down to "Emergency level" before I leave. I'll likely fill the jerry can (additional six gallons) before I return in 3 weeks.