I have always carried a couple of containers like this and filled them when I filled the tank. If you are boondocking in the same place for several days, two of these is 14 gallons and that is about half of the water capacity of many B water tanks.
https://www.amazon.com/Reliance-Products-Aqua-Tainer-Gallon-Container/dp/B001QC31G6/ref=sr_1_2?ie=UTF8&qid=1478050631&sr=8-2&keywords=reliance+water+containers
I also constructed a little submersible 12 volt pump from a small marine bilge pump with a 20 foot cord and two alligator clips and a short five foot hose. I clip to the batteries and pump up into the rig's tank. I carry my plastic cans in the shower area in transit and set them outside when I get to my campsite. This is not a direct answer to your OP question, but it sure lengthens the stay a bunch. When we had the Roadtrek we just placed them in the aisle at the back until we got settled.
PSW
2013 Phoenix Cruiser 2350
2014 Jeep Cherokee behind it
and a 2007 Roadtrek 210P for touring