If you look at the photo in the link that George posted, the back of the entire assembly, there is a "C Clip" that locks the actual city water backflow valve to the plastic housing. That part is available in black or white, manufactured by JR Products and sold on Amazon.
For the black valve, right now it looks like Amazon is out of stock, but you can buy the "round assembly" and pull the C clip to remove the backflow valve and then install that in your existing housing.
All you need to do is take the city water connection assembly off the trailer sidewall, pull that C Clip, remove the old backflow valve, install the new one, reinstall the C clip and reinstall the assembly back on the trailer sidewall. Be sure to use new butyl putty tape to seal against potential water leaking when it rains.
The backflow valve is the same in the circular city water connection assembly and in the larger gravity fill/city water connection assembly.
Here's a photo of the back side showing the C Clip. You MIGHT have enough clearance to remove the C clip without removing the whole assembly, but I've always had to take it off the trailer sidewall to have enough space to work.