Hi John - Just completed the install of the SeeLevel in our 2006 HR Ambassador this past weekend. The hardest part for me was gaining access to the ends of the gray & black tanks - Had to remove the white panel in the wet bay, but thanks to some excellent pictures and instructions I found on this site, I was able to do it with minimal difficulty.
I am not familiar with your hall monitor panel, but it is probably similar to mine - I removed the 8 screws and the panel came out easily with all the wires coming with it. I actually marked all of the connectors for wires and disconnected them, so I could remove the panel. I then used my Dremel moto tool with a cutting wheel and cut out the appropriate size opening for the seelevel display where the old tank monitor lights were. The Seelevel display looks like it was meant to be there, and I didn't have to cut any openings in the wall. I mounted a second seelevel display in the wet bay where the old monitor lights were - Again, used the dremel to enlarge the opening to the correct size.
It is so nice to have actual tank levels displayed

- my original system never worked well, and I was never sure how full the tanks actually were.
Good Luck,
Gary