If I understand what you are asking - you want to be able to use satellite when it is available, but when it is not available, use park cable. If that is exactly what you want, then yes, you will just have to switch the cable going into your trailer.
Now, if you are asking if there is a device that will switch the inputs, then I think you would need a diplexer (think splitter). I just did a quick google search and found this webpage with some items that will probably work. I am guessing others with better info will post soon.
Basically with the diplexer you will just connect the cable to the cable input, the satellite to the satellite input and a cable from the output side of the diplexer to the trailer.
I think then you would just have to turn off your satellite box to get use the cable from the park. Hopefully someone else will be able to confirm this.