Hooked Up To City Water And Then...

Brand new guy here. I hooked up to City Water (Well, actually) using a pressure regulator like I've been told to do in here.

Anyway, the folks I bought the Unite from, an XLR 29HFS Toyhauler, left all the Gray and Black Tank valves open and I figure it would be a good chance to purge the system.

So I ran the water and almost as fast as I put it in, it ran out. I was only running the faucets and the shower but every bit of water I put in seemed to drain back out through the gray (or Black) water tanks.

Or is this normal?? I found the HW Heater valves (under the stove top behind a wood panel) and the bypass is Off with the hot and cold water valves open.

Should I close the Gray and Black Water drain valves and fill it up? Why is water draining out of the Gray and Black Tanks, and the HW Tank, it would seem, when I have the hose hooked to the 'City Water' connection??

I closed the individual slide cutoffs (sorry, can't think of the proper name) and capped the main drain and will try again tomorrow.

Am I doing something wrong? Am I missing something?

If so, please help.

Thank you