I'll dig up some pictures when I can, but...
My freashwater tanks is a 60 gallon plastic drum located under my bed. It has two threaded bungs on it. All the plumbing is 3/4 and 1/2 CPVC. On the upper bung there is a tee. On one side it simple goes out and then 90's down with a ball valve. This is the air bleed. On the other side of the tee is a regulator (I'm currently searching for a better one), a ball valve, and a pneumatic quick connect like on shop tools.
On the bottom bung there is also a tee. One side is the supply to the bus with a ball valve. The other side has a ball valve and a hose fitting for filling.
So...how does this contraption work? To fill the tank you close the air valve, close the supply line, open the hose valve, and open the air bleed valve. When it gets towards the top I close the air bleed so the system pressurizes the air already in it. I can then close hte fill valve.
When I get to my destination I open the supply line to the lines in the bus. I also open the valve from the air pressure regulator. Compressed air is supplied by a 10 gallon air tank charged to ~120 psi, but is regulated down to probably 5 psi. My current regulator is not real accurate at low pressure which is why I'm searching for a new one. As you use water more air is supplied thus keeping the output pressure regular.
To remove the pressure from the tank I close the supply line (that's my first step for everything...don't know why), close the air supply valve, and open the air bleed.
To drain the tank I close the air supply valve, close the water supply valve, open the air bleed, and open the hose fill (with a hose attached).
To run off a city water connection I fill the tank most of the way in the normal fashion and then leave the hose connected. The air gap at the top of the tank helps to absorb pressure spikes, and eventually pressure equalizes and the tank stops filling from the hose...until a faucet is cracked.
Advantages of the system:
1. simplicity (it actually IS simple)
2. DEAD silent
3. no electricity required
Disadvantages of the system:
1. while I can recharge the air tank I have to do so off the bus' air brake system in the absence of clean compressed air. I'm not sure this is terribly clean, but I could if I have to. One charge of the air tank is enough for several fills of the water tank though....
2. it has required some playing with to work correctly. My biggest issue is with the tank
3. while I started the project to save money it has cost as much as a cheap electrical system...but is silent (important when the tank is under the bed!)
I'm not sure I would do this type of system again, but I could change it over to a pump at any time and for the time being I like the way it is. I also put unions in EVERYWHERE in the system which has proven to be invaluable. I can pull the water tank out in 2 minutes or less. That makes winterizing A LOT easier. I can just open the faucets and let the faucets backdrain. Then I drain the tank, flush it with a bleach solution, and store it vertically in the back of the bus. No antifreeze is required.