We also had a problem with very poor picture quality on the front overhead TV when we had cable plugged in. It worked fine with the VCR or DVD player. I used an old VCR as a test source and found that there was a problem somewhere in the coax between the cable socket and the distribution box up front. Since I had no documentation to indicate where the coax ran, I decided to by-pass it.
We'd already dumped the bedroom TV, so I ran a new coax from the back of the cable socket in the cargo bay up into where the bedroom TV had been and connected it to the line that had fed the bedroom TV. I then changed over the "bedroom" coax on the back of the distribution box to the "Cable TV" socket and we get a good picture now when on a CG cable TV system.
Initially, I thought of moving the Cable TV wall-plate to the forward cargo bay, just behind the driver's door and taking a new coax up through the bay ceiling and then alongside the driver's door, up the windshield trim and into the overhead cabinets, but the trick with the surplus coax from the bedroom was an easier fix.
Frank and Eileen
No longer RVers or FR owners