It is true that you need a straight coax type connection between the Sat receiver and the outside of your camper. The one coax on the side of your unit is for the purpose of cable TV as you have stated. This cable does most likely terminate on the back of the face plate closest to the over the air antenna. Likely this is where the button to switch between the over the air and cable connection is.
It is possible to use this coax for your Sat receiver but you would have to remove the coax from the back of the face-plate and terminate it directly on your receiver. So it would be difficult to switch back and forth if required. You may want to consider running another coax for Sat use as the dealer suggested, but the cost seems high to me. If you are a DYI type person you could run this additional coax for 40 bucks or less. Hope this helps.
Chuck & Sandra
2010 F350 CC 6.4
2015 Prime Time Sanibel 3601