First,
to the forum. we're glad to have you as a part of the family.
Now, as for your antenna situation. SOMEWHERE you'll find a small box with a switch on it. It'll look much like this and may be either black or white. To use the overhead antenna, you must turn power ON at that box. There is a small red or green light to indicate power. To use the cable, you must turn power OFF at that box.
Also, your TV must be set up properly. Go to the menu, set it up either for cable or digital TV, based on what you're RV is connected to, then do a channel search. Hopefully that will get your TV up and running.