Check valves can be tricky. DO NOT PUSH IN ON IT WHEN THE SYSTEM IS PRESSURIZED. It will unseat, and leak. Let the pressure off the system, and take out the screen in the city water outlet. Wiggle it around to see if it will seat. Pressurize it and check. If not, you will have to take it apart and reset the o ring.
Here's how you do it;
Remove the outside water inlet panel that has the water connection etc. on it. Mine has enough slack in the lines that I could get it out enough to get the hose clamps off. Try and save the sticky tape around the perimeter of the panel, it helps keep the water out.
Next remove the screen on the city water inlet and unscrew the plastic fitting on the back of the brass valve itself.
Behind the screen is a little plastic pin with a retaining ring and a spring behind it. Set the whole thing down and press down on the plastic pin so the plastic retainer pops off, save the retainer and spring. Now you can remove the pin from the back. It looks kind of like a golf tee, the o-ring will probably still be inside the valve, remove it and it will slip back over the widest part of the pin in the little groove. It doesn't seat on there very tight.
That o-ring is the part that gets knocked loose when there is pressure from the trailer side of the valve. It's meant to only have pressure from the street side of the valve. Re-assembly is the same as taking it apart, insert the plastic pin part making sure the ring stays put and then put the spring back in the other side and snap the retainer back on.
Then hook up the plumbing, re-install the panel and caulk around it with silicone.
It's not very complicated, just keep track of the parts and how you take it apart
Here is a helpful guide with pics: