Just to be complete, I have got the Magnum Display to work. I tried replacing the RJ-11 connectors, but no change. I found a wall plug with screws on the back that I connected to one end of the cable. I cut a spare 6 foot phone cord in half and used one end and tied all the wires together. I plugged this into a phone line connector and then plugged the other end of the Magnum cable into it. I used an ohm meter to test the cable. I had an open connection between all wires. This meant the wires were cut/chewed/broken somewhere. The only choice was to replace it.
I had a 200 foot reel of phone cable, so I snaked the cable from the rear compartment, along the left inside frame rail to the generator compartment. Then I remove the right pillar cover and feed the wire into the compartment in back of the meter. I used a #10 piece of wire to help go over the water and fuel tanks. I feed the wire from the front and taped it to the phone cable. Then I just pulled the wire to the next open point and repeated.
I added RJ-11 connectors at each end of the new cable. The four wires need to be mirrored at each end. Plugged in the cable to the Magnum invertor and the Magnum display and all worked. I just taped/tied the wire at several points along the way. It was a lot of work going in and out under the coach. I would say about 4 hours. So, it cost me $12 in parts and saved me $500 in labor
I hope it lasts for awhile.