Liam, As we have similar year coachs, I can say with some experience, your's has been modified.

the rinse port your picturing above is the black water tank rinse input. Should run to a small sprayer on the top of the black tank. Some one must have felt the need to disconnect it as the spray head often gets clogged with paper and stops working. I cannot see the rinse head on my tank, but I know its there. It was working up until about two years ago, when mine clogged up. I understand from Jim Keller, the way to clear it is to fill the black tank with rinse water to the top, drive around a bit to move the water, then drain it and try to clearn the rinse nozzel with water pressure. I've not tried that, yet!

Our coach has the same regulator as yours with a check valve well past the T fitting to the system and the fill valve. That means when we disconnect the city water hose feed, we get a burst of about a pint of water out of the system (that which is between the hose and the check valve). We have two 12 volt pumps in ours, both have direct feeds from the fresh water tank and they feed into a T system feeding the balance of the coach system. Way to tight a location to get a valid picture, so I wont even try. We also have "T'd" into the system a pressure switch which turns on the two pumps when the pressure drops to 40 lbs and turns it off at 55, a simple water filter, a small pressure tank behind the water heaters and a pressure guage. I've changed a lot of these items over the years and found most of them at Lowes or a plumbing supply house. The presure gauge was a bit of stretch as I had issues finding one that would mount in the hole provided for it.