Are talking about a Waste Tank Flush System Inlet connection?
OR is it the 'City Water' hose connection??
Sounds like you are describing the Internal Check Valve in the Hose connection assembly
If on a Waste Tank Flush System....no not needed
there is an Anti-siphon Vacuum Breaker Check Valve assembly inside a cabinet...typically bathroom room sink
A check valve at inlet will trap water in supply line that can freeze if not properly drained....that is why they typically do not have a check valve
If 'City Water Inlet' hose connection then YES check valve is needed to prevent backflow thru inlet when using the on-board pump
(Or use a 3/4" garden hose plug to seal it off when using pump)