I don't have a motorhome, but I'll take a stab at helping...
If it's anything like my 2001 trailer, the coax feed in the power cord compartment goes to the front wallplate that has the coax connector for and 12V
lighter plug for a TV. On the way, it passes through a wallplate down below inside the credenza. This plate has two coax connectors with a short length of coax joing them. This is what you remove to install a satellite or cable receiver. One connector goes to the sat in port and one goes to the TV out port on the sat receiver, and it's important to connect the correct port to the correct connector on the wall plate. The TV out port from the sat or cable receiver is fed back up to the wall plate that has the 12V
and TV cable outlet (not to mention the push-button switch and green light that tells you when the VHF/UHF amplifier is on).
Once you've replaced the loop below with the sat or cable receiver, the little button on the 12V/coax panel that turns the power on and off to the VHF/UHF antenna amplifier also selects whether sat/cable or VHF/UHF is going to the connectors on this plate in the front of the trailer and one in rear of the coach for TV. When the amp (and green light) is on, VHF/UHF is going to the coax connectors in the living room and bedroom. When it's off, sat tuner output is going to those places.
Airstream does not use the VHF/UHF antenna in port on the satellite tuner to combine the signals. It's all done through the switch on the wallplate. The wallplate is not designed to handle the GHz signal from the dish, and I'm not sure if it would hurt the amplifier if that was on. It needs the TV out from the sat receiver.
There's an on-line Winegard instruction manual that shows exactly how Airstream wired it, and I looked there for it but can't find it now.
Hope this helps,