help replacing presure releif valve at city water connection

[Cstoned]

City water connection is pressurized, but I have no water inside trailer?
Water system has been fine for past 5 years living in my 1994 Excella 30' with mid bathroom.
Water supply has been intermittent for a month and now no water at all.
online research has brought me to the pressure relief valve located behind a small panel at the city water connection location.
I have removed the four small screws securing the device to the panel.
How the heck do I dis-connect the device from the brass nut fitting ?
Not enough slack in connected water lines to put a wrench on the brass fitting to remove pressure relief valve, let alone put a new one back on.
I appreciate any ideas that help me with clarity on this fix it.
Thanks.