I am not a hydraulics guy so forgive me if I am off base.
As I understood it, the pressure source (pump or city water connection) providing the highest pressure will supply all the volume used.
So if the city water has a higher pressure, the shurflo pump will run in a "no flow" condition (as such it does NOT contribute - add- its pressure to the system.
However, as pressure fluctuations in the incoming city water occur (taps open/close or high volume use on the campground supply pipe, momentary pressure drops at the output side of the pump can cause FW tank water pressure to momentarily exceed pipe pressure and open the reverse flow check valve; thus allowing the returning city water pressure to be forced back into the pump potentially damaging the check valve and fill the fresh water tank even if the pump is subsequently turned off.
I have been told that for the above reason, he pump should not be powered when city water is connected to ensure the check valve remains solidly closed while the lines are pressurized with city water pressure.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW