Shadius, I just went through the task and found this thread to be very helpful. A few suggestions.
Is your water fill unit old style (city connection on the left when viewed from outside) or new style (city connection on the right)? If it is new style then you have the option of easily replacing the complete hatch unit. If it is old style then you can convert it but with some fiberglass cutting and replacing the fill lines.
Looking from the inside, is there a swivel connection, easily unscrewed by hand, on the water line? Or is the water line pex crimped to the city water inlet? If it is pex crimped then you will have to cut that line off to remove the pex barb adapter and get at the back of the check valve. In my 2010 trailer it was crimped, so when I reassembled everything I used sharkbite type fittings with a swivel connector to make it easier next time.
Drilling out the rivets to remove the housing is quick and makes it easier to work on. I ended up going that route.
The actual o-ring replacement isn't bad.
Good luck, and I hope you find that you have the new style unit and a swivel connection.