I second PEX, because it is easy to work with, is fairly flexible, freeze-resistant (not freeze-proof, but more forgiving), and there are many ways to join PEX tubing together. Only fittings I'd be leery of (just from reading the Airstream forum) are Sharkbite ones (there are better) because only one O-ring separates you from a leak, and there are others that are reusable like Flair-It or Seaquest which do a better job of securing the pipe onto the fitting, and use pressure all around the pipe to ensure a seal.