First of all, congrats on your new coach and welcome to the FORUMS!! Hope you have many fun, safe and trouble free trips with it!!
I would first check the position of the valves in the compartment where the water pump is. You do not need water in the tank when hooked to shore water. The backflow valve is for the city water is just inside the connection. When you take the city water hose off, you will see a little black ball. That is the back flow valve. If you press it, see if it pops back. Make sure that you let the pressure out first or you will get wet (LOL!). I don't think that is your problem because, as it name implies, it is a back flow valve and its purpose is to prevent the water from flowing back into the hose. If you have water pressure from coming in, the valve is pushed in (open) and lets the water enter the coach. Also, you might want to make sure that your outside shower valves are turned off. There is a hot and cold water handle, like you would have in your bathroom at home. Make sure that those are closed. You could also turn off the valves on the manifold for the outside shower.
Tom and Margaret
2014 Berkshire 390bh-60