Brad,
I will try to get a wiring diagram for the entertainment system for the Sundance. If we are able to get it, it will be more of a wiring plan and less of an exact location of wires sort of a plan. But it will give you a much better sense of what you have to work with.
Depending on whether you are using Dish or Direct, you may actually need 2 satellite feeds to the dual tuner receiver (Direct TV). I believe the Dish dual tuner receiver splits the satellite signal inside the receiver, thereby requiring a single feed. That would be good news for you.
If it exists, using the cable marked "cable" as a back-feed is a great idea. Like HT said, who knows how it is run and there is likely a splitter in the line that will need to be removed. Take your time, explore your RV, remove panels here and there, learn a lot about what is where in the belly of the beast and you will likely find all the you need.
One you do trace out that "cable" line from the LR to the UDC and remove any splitters, you will then need to connect that line to the line that feeds the BR TV. You may have to remove that line off the wall plate at the BR TV if that is where the antenna amplifier is (white wall plate with red led and power button on it).
Not impossible at all. Not easy, not fun but doable. That said, the job is not for everyone. Not technical, just very tedious figuring it all out, finding all the routing, splitting etc. Best of luck.
Jim