I thought it might be worth copying som estuff about 2 way valves over to this thread.
I wanted to be able to select between using an external water source and the water tank. To do this, you can get a two way valve (often called an L port valve). Just sticking in an L port valve is not enough as you could contaminate your drinking water so you need to use some check valves. They only let water flow one way so there is no chance of any cross contamination when you switch the valve over. So here is a smaller version of what the council wanted when they connnected town water to a house I owned once which was just on rain water:
Believe it or not, you are looking at about $90 worth of fittings. The shiny round fittings are the check valves.
The two tails pointing to the right are the inlets. One for the tank and one for the existing shower inlet I already had. The other goes to the tank. The tail pointing to the bottom ofthe pic is the outlet that goes to my pressure pump. So where to fit it? There looked like to be a good place just under the right hand side headight that would be easy for me to reach the valve. I used a couple of copper saddles to hold the fittings on. I have found the box of self-drilling galvanised roofing screws I had left over from building my shed to be great for mounting this sort of stuff.
It really would have been better if I had bought another brass elbow for the bottom tail so it also pointed to the right as well. No matter, I just used a low density poly irrigation elbow:
Some people reckon plastic fittings are the way to go on a vehicle as they are nice and light and thus stand up to vibrations well.
The white hose is certified drinking water hose so there should be no taints in the water. Be sure to make up (or buy) a drinking water hose to fill your tank with so the water does mot get a plasticy taste as you fill it which can occure if you just use ordinary garden hose.
Skog raised some issues about using galvanised screws and copper saddles I used here being a metal fire risk. Hopefully he will chime in and explain this more fully as his comments were not clear to me.