I originally got my water tank refilling idea from BradW's handy 7 gallon portable water filling solution a long time ago.
7-Gallon Water Jug With Pump
I liked this project quite a bit, and built my own version of it after a fashion, substituting a 10' standard RV hose for the black marine hose that BradW used.
However, as handy as this was, doing this 4-5 times in a row to refill the onboard tank got old, really fast. Not to mention, I could never get the pump to stay in put, it would always try and turn when you unscrewed the lid from the jug.
So, a long while back, I started a couple threads asking about details on those big 30 gallon plastic barrels. After gathering the information I needed, I wound up waiting a couple years until I eventually had built my front cargo rack (next project thread to be posted).
I did a search on Craigslist and found a used Peach Flavoring barrel from Kerry Foods for $25 that came with twin bung plugs that had 3/4 threading in them for the attachment of pipe.
I wound up building this, using the pump I had originally been using with my jugs:
(Click Image to view enlarged)
The tank has two ball valves, PVC because I couldn't find an affordable brass one. The upper one is the breather and is raised up above the top of the tank to facility a full fill. The lower one has a male garden hose thread on it, which the modified pump assembly attaches to.
I use a double-female coupler to fill the tank via regular RV hose, simply screw on double female, attach RV water hose in normal fashion, then open both ball valves and turn on the faucet. Your standard water pressure is more than sufficient to bottom fill the tank 30 gallons. When it reaches full, I close the bottom ball valve and then turn off the spigot.
The other advantage of the auxiliary is I can take it, an RV hose and a water thief to the filling spigot in most USFS campgrounds and fill the tank while leaving the camper behind at camp, which allows me to refill the tank from bone dry in one trip.
With both tanks full, I start out with 60 gallons of fresh water to work with. This came in rather helpful while I was touring Whidbey Island with my wife-to-be, Dawn. The Washington SPs shut off the water spigots at the campsites during the winter to prevent freeze damage. The only water supply is the potable water faucets back at the dump station. Depending on the park, the dump station is a mile or so away.