I use a potable water hose that I connect to the water faucet... I also have a little device you connect on the hose and it tells you how much water (in US gal, Imperial gal and liters) has been used...
So I know how much water I am putting in before hitting the road...since I limit the weight by carrying only 2-3 gallons just to cover the road washroom stops
2014 JayFlight 28BHBE (replacing a 2009 Kodiak 25QS)
2011 Sierra SLT CrewCab 4x4
1200/12,000 Equal-I-Zer Hitch