I got all of my stuff from a farm store as well, Tractor Supply to be exact.
Things you will need:
Water Tank ( I got a 65 gal, but depending on the size and weight limits on your trailer.. you get what you think you need.)
Utility Pump ( I got a 3.8 gpm pump, works GREAT)
Fittings
A small hose
A marine battery or some other kind of power source.
Picture of a pump that will work for your needs
They have these pumps at home depot, lowes, sutherlands, etc.. probably find them in the plumbing dept with the sump pumps.
Youll need female and male hose fittings, 6-8 inch hose, and a shut off valve.
What I did was put the shut off valve on the tank, then ran my house to the pump. You will also need a longer house of course to attact to the opposite side of the pump so you can get water.
Most of these pumps come with a -/+ wire, so you will need some sort of power source to make this pump run. I got a battery and connected the wires to it and works great...
Good luck, hopefully I helped lol
In this picture, I have the shut off valve connected to the tank, the female hose fitting connected to the small hose and the other end going into the pump....
The wires running to the yellow box are connected to a battery. Black with negative, Red with positive.
You can see where the hose from the tank is connected to the pump.. the yellow hose will be your water source. This pump provides enough water to run my 2600 psi power washer so you should be perfectly fine....
Overall look on the front of the set up.
Hopefully this is helpful, I had a heck of a time researching this and getting help from people to make this set up possible. LoL someone needs to do a how to on building your own mobile wash system haha
Good Luck!