There's a tiny push button located somewhere inside or near the entertainment "in" cable location, that toggles between cable and antenna input, I believe. It's inside the coach, not outside. I'm not sure where it was in our 2016 Sunstar, but it may have been in the forward overhead storage, near a little green light. In our Navion, it's behind the galley TV, and I found it by accident, while having the same snowy picture issue.
Look around for a little push button, and try it. We had the same problem back in February when we used a c/g cable connection, instead of the OTA antenna. Gotta switch it over.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)