I went with copper , be sure to use the thicker grade of pipe available at big box stores.
I used braided stainless sections above the rear bumper to connect four quadrants of pre fabricated pipe--- I then fitted with solder sweats , if I felt comfortable with applying heat in the confined space-,- compression fittings if not. I also ran braided stainless to the water heater and sinks to avoid sweating solder in confined, fire prone areas.
This makes isolation of four zones easy to test and replace if necessary,
1 porstide Hot
2 portside Cold
3 starboard Hot
4 starboard Cold
Be sure the heater is properly ducted for cold weather survival.
I could just about pre fabricate and mail these out. I had fun making them and almost made spares!