No check valve on the fresh water fill for the supply tank?
Ok, first a question.. On most RV's that have a hose connection you can fill the fresh tank from the hose connection is the standard city water connection and that is checked.. The other connection is a gravity feed meaning it is ABOVE the top of the tank and it is not checked, it is also like 1.25 inches diamater (swimming pool vacuum hose size).
Does your RV have a SEPERATE fill fitting? Or are you talking about the flush fitting for the black tanks?
That one should have a vacuum breaker, well above the tank (often in the bathroom) and fresh water (only) will leak back when you disconnect. However only what is in the line up to the vacuum breaker.. From there on it's checked.
If you have like a Tornado.. then there is a spring check in the tornado, but the supply line may leak back and due to the design of the line this may be slower.
I have the tornado, but fed by the original vaccum broke line so,, Well, it's complex. (I like complex sometimes,, And the hose I feed it with has a second spring check and a second vacuum breaker,, Belt and suspenders yes. Effective yes. I'm not paranoid, Germs are out to get me... And you.. And everyone else.