2017 171 RPod water leak

What is likely happening is that the check valve in the water pump is not closing properly. The fix for that is to shut off the city water and then cycle the water pump several times to clear the obstruction. Most of the time that seems to work. I have not had it happen, but I've read about it often enough. If that does not fix it, then it may be that the pump needs to be disassembled and checked/cleaned or it might be a good time to replace the pump with a newer model.

Look where you connect the hose. If you disconnect it, underneath a screened washer you will see a little piece sticking out of a piece shaped like a Mercedes symbol just inside the connection. That is the check valve. Press it, and you will feel it move. If there is pressure from the pump, you will have water come out of the city water connection. 

The gray tank is where shower and sink water drain. The black tank is where the wastes from the toilet go. There is no interconnection between them and the fresh water tank. Now, if you are using gray to describe the color, then the fresh water tank is translucent white which may be what you are describing as gray. The gray and black water tanks are both opaque black in color. 

Edit:  Revisiting your first post, if this valve is where you are getting water, then it is likely not the city water connection you have connected to but the black tank flush. It looks the same, but the city water connection is the one next to the fresh water tank fill port. If yours is like mine, the black tank flush port is the one on the door side of the RPod. The tank that is filling is your black water (waste) tank. You don't want to do that. It is for flushing out the black water tank when you dump it. If you leave the hose connected, your tank will fill and you will likely have a very unpleasant fountain coming out of the top vent on your RPod.