There are several possibilities, and among those possibilities are:
- Water pressure regulator. Typically found very near where the city water enters your coach. It can appear to be a leak under your coach when there is a high pressure condition in the park where you are staying, if sand, sediment or hard water deposits have blocked the valve open. It is not uncommon to need to replace this valve on a vintage coach.
- It is possible that you may have one or more low point drains that are partially open and this is appearing as a leak. Looking immediately above the point where the leak appears -- you may find a low point drain - - try turning the valve in the opposite direction from its current position - - that may stop the leak.
- If you are using your air conditioner and its is an original Armstrong Bay Breeze or another of the Airstream supported brands with condesate collecting tray and drain, it may very well be the the drain pipe for the condesate from the air conditioner.
- The last possibility that I can think of would be the expansion overflow from your fresh water tank. It isn't uncommon to have some expansion occur in your fresh water tank if it is filled to its capacity in the cool of the early morning . . . . then in the hottest part of the afternoon there is enough expansion that there is some overflow from the expansion tube -- particularly if you haven't utilized any water from the onboard supply.
Good luck with your investigation!