You say you ran it off the antenna and it was fine. Was this a portable antenna directly connected to the TV, or the roof antenna with the signal running over the RV coax to the TV?
If it was the roof antenna, then you've pretty well eliminated the coax in the RV. If you have a selector switch that you use to switch between the antenna and the dish, that switch could be the issue. Try moving the dish wire to the input that the antenna is currently hooked to.
If you were using some sort of portable antenna on the TV, then try running a piece of coax from the front of the rig all the way to the back. Just run it down the hallway as a test. If things improve, then the coax in the coach is bad.
