Unless a PO has removed it, all Airstreams of that era came with a very good built in Watts pressure regulator. Of course it is 40 years old now, and could have packed it in and needs repair or replacement.
Look where the water comes into your rig, and you should find a dome shaped copper colored thing, about 6" tall, with a pipe in, and one out. That is the Watts regulator. It is better than any external regulator, assuming it is working right.
Now, actually where was the leak? Even with a pressure regulator not working the plumbing should never leak up to about 100 psi, and probably higher. I would look very carefully for the leak, rather than worry too much about the pressure. A leak generally will still leak with less pressure, just leak less. The leak itself is what needs your attention first and foremost.