Hi...
In the tank dump area you have that white plastic wall. The tanks and probes are right behind that wall. The wall is in two sections and many screws. The top part comes out without an issue, the bottom section is harder to get out.
I replaced all my probes in the black and gray tanks with these...
http://www.rvprobes.com/home.html
Have not had an issue from that time! You will need to drill new holes right next the to current ones as the ones GS used are not removable. Then use the same wires. Very simple to do. Not to worry. (Just make sure your tanks are empty!!!! LOL) THe worst part IMHO was getting the white panels off and back on.
NOTE...their are two differnt probes, one type for the gray tank and the other for the black. One set of 4 for each tank type. They are not cheep...IMHO...but I am SO HAPPY I took the time to do it with these.