You should be able to get away with a simple AB switch in your AV compartment that switches the cable input to either the Dish box or your current AV selector.
From the dish box, use the coax output and feed that back into your AV selector into an open port (assuming you have one. )
Keep the external cable from the dish to your external (cable) port as short as possible since the cable used in the coach is likely RG59 (the thinner type)
Finally, get yourself one of those signal meters to use when pointing the dish since it is doubtfull you will be able to hear the signal tone from the TV inside the coach.
This is how I hooked up my mother's Safari...it should work fine for you as well.
2004 Ultra Supreme Class A 30'
- Added hard-wired Pro-Sine 1000 Inverter
- Converted batteries to 4x6V from 2x12V
- Added Bully air-horn
- Added Safe-T-Steer