Rick, when I did my Series 60 conversion I had to make a lot of tubing parts (air and water). I used exhaust tubing. Often the enlarged ends of the tubing will let you mate two different sizes of hoses.
One thing to consider, if you use any form of metal tubing, you should put a bead on the tubing to assure that the hose will not come off. You can see some of my plumbing construction (including the forming of the beads) on my bus pages at:http://www.rvsafetysystems.com/busproject5.htm