black tank flush valve

Sorry. I have a 2003 Damon intruder. It's a 36 foot gas. The flush valve I'm talking about is directly below the city supply inlet in my wet bay. When I connected the parks supply to the flush valve no water entered the tank. And yes, I have a fitting similar to the detail above under the sink in the washroom.....i was wondering what that was for

Some information on what brand motor home you have would help with the diagnosis of your issue.

It should unscrew from the back a new part is about $15
and no the black tank will not drain out check the diagram above

Yes. Mine seems to have a pressure switch of some kind right at the inlet. It looks to have a spring. How do I remove it and if I do will it let the contents of the black tank out?

Some if not most exterior inlet fill have a pressure switch,I had to replace one on my black tank flush

There is a check valve - vacuum breaker that can go bad. Easy to replace if you can locate it behind a panel or inside a cabinet..maybe under the bathroom sink in the vanity...

Here is a basic diagram. All of them are similar.