Typically there is a pressure control valve at, or near, the fresh water inlet. It may be a part of that inlet fitting. The Airstream internals are designed for something around 40 psi. City water systems cal deliver 70 psi or more.
Where in the picture is the leak? Is the nut on the back of the valve tight? Is it or the valve body cracked? Is it leaking where the screws hold the back of the valve on?
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