I'm guessing here, but I think the reason these were installed was to protect the 12V
pressure pump and perhaps the shower hose from too high a city water pressure in case the regulator failed or got stuck. The copper lines would easily handle full city water pressure or higher - I've routinely put 200 psi through similar copper lines on our steam launch. The water heater also appears plenty robust enough to handle 100 psi or more.
In any case, limiting the water pressure to 40 psi or so is a good idea, and such valves are readily available as others have noted.
One thing that would be an improvement in the water handling on Airstreams are fine screen filters protecting the pump intake and the city water inlet. Debris in the water line (rust flakes, etc) will raise hob w/ check valves, water regulators and the like; such simple filter really make pumps, etc. more reliable when the water gets "chunky".