I replaced my original TV antenna panel on the inside of the trailer with an amplifier, which includes a 12V
outlet for a small TV. This new amplifier panel has inputs from cable, or from the TV antenna.
Then, I ran a coax cable through the closet down to where my water heater lives. There was enough space next to the water heater to drill a 1in hole through the inner skin, then another smaller hole ( i forget the exact size, the coax connector determines this) through the outer skin. Now I inserted a coax connector through the outer skin and mounted it with a simple stainless washer and nut, with the longer end of the coax connector sticking towards the inside.The connector has a flange on it, perfect for this purpose.Apply some par bond to seal the flange. Just screw the coax cable on this connector, the other end to the TV amplifier, and as Dan Brawner suggests, a push of the button selects either cable tv or antenna. All you see on the outside is a small threaded stub somewhere near the water heater. I put a small threaded cap on it when not in use. I believe that this is a very solid and permanent installation.