The check valves are there but they are internal to the water sources.
The check valve on the water pump is INSIDE the water pump. It prevents city water from spinning the pump backwards and filling your fresh water tank.
The check valve on the city water connection is located inside the city water connection behind the screen. If your "carefully" remove the screen you will see a while plastic tab pointing at you. It is spring loaded and should move freely with no water pressure on it.
For completeness, there is also a check valve "in line" with the turbo flush, if installed. It is located under the bathroom sink in my 5th wheel.
That check valve prevents a full black tank from siphoning out the hose connection for the flusher.
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