Confucious say "you must close all valves in system".
Airstreams are equiped with drain valves. All drains must be closed to pressurize and prime the system.
They are almost always at the underside of the galley and often at the rear compartment.
Some folks install bypass valves for the water heater and there is a drain at the water heater.
My water city connection will flow with water when the pump is running. I have a cutoff valve at the city water connection that has to be closed.
Long story short, check all valves and insure that they are closed.