Truly, the best thing to do is to use the outside to TV on post connection cable to troubleshoot. First, connect it to the feed to the front TV right at the back of the tv, run this cable down the pathway through your rig to the bag TV and plug it in. Does it work? It worked fine on the front, so why not work now? Accept that as it is, then disconnect both ends and connect to the feed that is feeding the back TV and run the long cable to the front TV. Does it work? I am thinking you are going to find the back TV is not set up properly, but the feed is just fine. Certainly, if you can run a TV feed directly to the back TV from out a window, and test it that way, that is another method to prove it out.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel