I use a food grade 55 gal drum. The drum has a large rugged plastic flange around the top. I installed two metal straps across the top of the drum by drilling holes in the plastic flange, bending the metal straps to fit and bolting the straps to the drum.
I then mounted a 12 volt Shurflo demand pump to the straps. (The pump I selected can also be used as a backup pump if needed) Attach a small length of potable garden hose with a male hose connector to the outlet side of the pump.
Obtain a brass garden hose thread to metal pipe thread adapter. The adapter should have both internal and external threads on the metal pipe side. Typically the external threads are 3/4" and the internal threads are 1/2".
For the inlet side of the pump - select one of the drum openings to feed the pump. Select the smaller 3/4" inch opening if you have one, otherwise select one of the 2 inch holes and fashion a plug with a threaded opening, then screw the garden hose adapter into the 2 inch plug.
Using the internal threads of the garden hose adapter-screw in a plastic or brass barb fitting, then attach to the barb a length of potable hose that has a female hose fitting on one end. Put a small hose filter into the female end and then attach a small length of hose Which has a male fitting on one end and a slant cut on the bottom end. The length of this assembly should allow the hose with the slant cut to reach the bottom of the barrel. This assembly will now allow you to use the barrel in any vertical or horizonal position, but make sure you strap the barrel down. I like to find a way to add some additional brass fittings to the bottom of the assembly to add weight and encourage the hose to always seek bottom.
Screw the assembly onto the drum hole you have selected. Attach a length of garden hose with a female hose fitting to the garden hose adapter and attach the other end to the inlet of the pump.
Add some wiring to the pump as desired. I like to use a 2-pin flat connector to feed the pump. The 2-pin connector will plug into a 4-pin flat connector and I will sometimes use my trailer harness to power the pump off the taillight circuit. I also have adapters for other uses that use the other end of a two-pin flat connector.
Pump is now ready for use. I will either fill the fresh water tank, or I can connect to the RV fresh water inlet and pump on demand right from the drum. I sanitize the drum whenever I sanitize the 5er.
Total cost = cost of the drum plus $20-$30 in hose and hardware. I actually have two drums - one I use for spraying. The second drum uses PEX tubing, but I also like the garden hose approach. They each have their place.
Enjoy ...
