Y adapter for fresh water at park water supply post?

Can't be any more of a risk than connecting the flush straight to the spigot. As mentioned, be sure the flush side has a backflow device. Or, just figure any cross contamination will help build your immune system :)

I would guess that those gray water flush connections have a backflow device, or else there would be a cross connection problem.

Pick up a hose bib backflow along with the Y.
Disconnect the grey hose when not in use.

I attach water pressure regulator on CG faucet then a short hose.
On end of short hose I install a shutoff 'Y'---------one side is in-line water filter then potable water hose to city water connection/other side of 'Y' is a garden hose for general use------flushing of waste tanks/washing rv/hosing down etc.

Seeing as water tank flusher has a backflow preventor and 'y' is always pressurized not concerned with cross contamination

That IS a cross connection, but the probability of actual contamination is minuscule.

Bruce

No, unless the gray hose connector is rife with Ebola.