I assume you've done the basics like turn off antenna, set the TV to cable and search channels. If so, this problem has been discussed in depth on this forum. You may need a converter since many of the TV's Thor used (Seiki brand) does not have the proper set up to receive the new signals from some of the campgrounds...or replace the TV's with new Samsungs.
If you have an external TV antennam you have a power supply for the antenna. The power supply must be off to use a cable. The location should have been covered during your PDI. Any dealer can telly you where it is located.
Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow
The switch for the power supply described above is usually in the same panel the the coax from the TV is connected to from the antenna. Sometimes the location of that panel makes seeing the little push button switch hard to see.
2018 Freedom Elite30FE
Current toad - 2019 GMC Canyon 4WD
Former toad - 2004 Jeep Liberty