I have the same motorhome except in 35 foot and I'm doing the same thing. Somewhere in here I have a thread on it. I'll try and find it and post a link. I bought a 35 foot HDMI cable and I'm going to route it attached to the ceiling. Home Depot carries plastic channels designed for cables. I'm going to attach it to the ceiling and just run the cable straight back to the TV.
I didn't make this decision easily. I researched the wireless transmitters and the plus and cons and also looked at various ways to try and route the cable in some other manner. There was just no simply solution to this problem. The ceiling already has vents and lights and various things hanging from it so I figure a plastic channel running the length of the motorhome isn't really going to look that bad. I have seen other photos on here where people have used the channels for various things and they don't look that bad at all. And if the cable ever goes bad, just snap the cover off the channel and pull the cable out and replace it.