The current Escapees magazine actually had something useful in it - the LONG title of this post.
I posted about our solution on our private blog back in 2009. I edited it today and here it is (http://kokoandpelli.blogspot.com/2009/04/what-to-do-with-spare-water-pump.html).
We, too, carry 3 5-gallon containers of water. We don't have an extra water pump, so we bought a drill pump that's used with an electric drill like this one:
Amazon.com : Lanchez 225GPH Drill Pump for Electric Drill with 3/4 inch... (https://www.amazon.com/Lanchez-225GPH-Electric-Suction-Discharge/dp/B00ZTRKNT2/ref=sr_1_1_sspa?ie=UTF8&qid=1514661475&sr=8-1-spons&keywords=Electric+Drill+Pump&psc=1)
It works very well.
In a pinch, gravity can work to put some more water in the tank too.
I have taken a full sun shower, hooked it high on the rear ladder, and let gravity drain the shower bag into the water tank (via a hose). It take very little effort and you can get 10-15 gallons in without much hassle if a faucet is nearby.
Most frequently I do this after settling at a campsite only to find that there isn't a threaded faucet within hose distance but the campground does have water at a bathhouse or pot washing station. It may or may not be correlated with longer showers by my offspring.
Rich
'03 MB in NC
I found this manual pump helpful when we had our Hi-Lo trailer. It's also a great workout! 8)