The first thing I would do is look at the three or four places that cables normally terminate, outside cabinet with the shore hookup from the pedestal and/or satellite input, cabinet above the dash where the B.O.M.B. and power supply for the antenna is, behind the front TV. In my case there is a cabinet above the driver with Sat. hookups and one over Passenger for the sound system and antenna and DVD player is located along with the B.O.M.B. The input for the Sat comes directly into the coach thru the roof but many have a cable hookup for a Qube or other remote Sat. dish.
If there is a unattached cable or if you find a cable connected to the B.O.M.B. that is not for the TVs you have that could be it. Look first, then, buy a wire tracing setup. They run from $20 to $20,000 bucks. Some RV shops have them and will trace it for you.