Define "tried everything".
Comcast uses encrypted cable in most locations which will require one of their tuning boxes for each TV and may not work going through the standard Winegard power injector/splitter most likely installed in your RV. In any case, the amplifier power will need to be off for the cable signal to pass through.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad