Unfortunately no one seems to have had the same problem and provided a solution. However, luckily my friend Tom has the same trailer and had removed the complete compartment. He came over and told me what to do.
Actually it was pretty simple, but very tedious as half the work has to be done blind and by touch.
First, I used a 5/8" open wrench to slowly turn the nut on the inlet on the right. Keep the screws in the inlet so it does not turn. When the nut comes off, remove the screws and remove the inlet. This then allows the backing plate to be removed and thus gain much more access to the inside of the compartment.
Whereas initially I thought I would have to cut the copper collar around the pex tubing which is connected to the water inlet and check valve, luckily that was not necessary. The water inlet is connected to a nut on the pex tubing. Note this nut is not 5/8" but rather 15/16". Using a 15/16" wrench simply (ho ho) slowly turn it off, again leaving the four screws in the water inlet so it does not turn. Once the nut is off, remove the screws and the water inlet comes out.
Now the problem and tedious work starts. Screw on the new inlet onto the nut, holding the nut in place with the wrench. This all has to be done by touch and is very difficult as three times I had to remove the inlet as it became cross threaded and the water inlet threads are plastic. Using a mirror and a light helped but it was not easy. (see the various pictures).
To ensure it was sealed properly I first turned on the water pump - no leaks, then hooked up a water hose and again no leak. Then it was a simple job to reconnect the right inlet and put all the screws back including the backing plate on the right.
It will be great not to have a water leak again.
If you have to do the same and have any questions send me a PM.
I think I have to include another post with the other two pictures.
1994 34' Limited
2004 GMC 2500 HD