A few observations (and guesses)...
These both look to me to be the same type of water fill port. The second looks like an older version of the first, and with the sealing cylinder removed, or broken and missing in action.
Why two of them is a mystery, but one with clues on the other side of the wall. Perhaps when the old one broke, they just added a new one and re-piped to it. Perhaps because it was hard to get to the piping at the old one.
The gunk over the vent on the old one looks to me to be unintentional, like over spray of whatever that yellowish brownish stuff is.
Lastly, you might want to think twice about your desire for a city water connection. It will require a new fixture (as pictured by a few posters above), a new hole in the shell, and some plumbing. I would sure as heck keep a functioning fill port because you never know when you wont be able to hook up a hose at your site. It is most of the time for me.
Connecting a hose full time to my rig has rarely worked well. Usually there is at least a small leak that makes a muddy mess outside the camper. And there is also the chance that the hose will fail at any time and a non-stop flow of water will result. And the check valve on the city water port can fail (as did mine), so that if you are using on-board water, it will all come out the port when the plumbing is pressurized.
So I fill my tank and use water from the tank without a full time connection. If I need to fill again and there is water at the site, then I run the hose to the fill port, and stow the hose after the tank is full. Otherwise I drag out my 5 gallon potable water can.
This also helps me manage my waste tanks since I have a good idea how much water I have used when the fresh water tank empties.
Now if you are going to often have a full time sewer connection at your site also, then city water makes sense. But if not you might try only using on-board water for a while and see if that works for you.
Also get one of these.. makes using the fill port easy.