Hello old hen
If you're hiring the work out, then the Airstream service center in Jackson Center would be able to help you with that. It is adjacent to but mostly separate from the Airstream factory, from what I understand. By all reports, their capabilities are considerable and their rates no higher than all the other places (I've never been myself, it's too far).
The "leak" arrow in your photo points to the check valve portion of a combined check valve and pressure regulator. There is an access plug there for cleaning and service of the check valve swing assembly. There are three approaches to the repair:
1) Remove the check valve and regulator assembly from the trailer so that a bench repair may be made. This would allow wrenches of any size, impact tools, and torches to be brought to bear, as necessary.
2) Repair the check valve plug gasket in-place as you are attempting to do. If I were doing this, I'd try to get a 6-point socket wrench in there on the end of a long breaker bar. If there isn't room for that I'd try the box end of a combination wrench.
3) Remove and discard the assembly and replace it with a modern equivalent.
The check valve is necessary but the regulator less so. It's rare to run into campgrounds that have a problem with high water pressure -- I've done so once in all my travels. Easiest thing to do when encountering that, except for a long-term stay, is just fill the freshwater tank from campground water supply and run the pump.
To learn to see below the surface, you must adjust your altitude