So if I am understanding this right, the aux tank is for increasing the volume of water in the system and not an expansion tank as such? If I have an open system to the trucks coolant system I can position the aux tank somewhere that is a little harder to get to and use the trucks expansion tank for filling the system and expansion?

What valves did you guys use?

I am thinking of using insulated plastic poly pipe the same stuff that I use at work in plumbing solar hot water systems in new house builds, Has anyone had any experience in plumbing a camper with this stuff?