The very fact that it works when wired directly and does not when wired through the coach should be all the information you need. No one can be sure what is or what is not causing the issue and there are so many variations of wiring and wiring types even RV to RV by the same manufacturer.
I would suggest finding a new route into the coach where you can run high quality coax from point to point.
What I post is my 2 cents and nothing more. Please don't read anything into my post that's not there. If you disagree, that's OK.
Can't we all just get along?