The pump won't (shouldn't) run when you are hosed up to city water because the city water pressure in the trailer is higher than the cut-in pressure setting of the pump whether the pump switch is on or off.
So like Redhorse said, there is no need to have the pump switch in the on position.
Rye's onto something with the water conservation valve on the shower head. Mine has a metal lever, some may have a spool valve with a push button.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.