Ok dudek
Firstly, how fast do you want to produce water? If you only want to produce 500ltrs a day then something like a 300gpd ro-man should do. Check your water pressure first, if its below 50psi get a pumped system.
If you want to produce water much faster then a 4040 is the ticket.
Next you need to decide how much water you want to store and how much you can actually store - if you have room then a second hand IBC tank will be cheap and hold 1000ltrs or more - if you don't have the room then a couple of water butts will have to suffice.
Finally, getting the water to the van/trailer - if the tank is on the ground you will need a submersible pump, 100ltrs/min as a minimum. Attached to this you need transfer hose, work out what you need and then add another 5mtrs on just in case.
You will also need a selection of fittings but decide first if you want a 4040 or a small Ro-man unit. Any good supplier will throw in all the fittings you need.
If you're stuck at all e-mail me
j.mills175@btinternet.comregards