I recently acquired a 1997 Fleetwood Tioga Montara. When I turn on the propane valve at the tank I hear a hiss. There is a small threaded female opening (1/4" approx) in the first component after the valve (pic attached). The diagram in the manual does not show this component. When I block it with mt thumb I feel gas pressure and the hissing stops. What is this and should it be plugged?