When I asked pretty much the same question, I wanted one signal source to go to both sets. I intended from the onset to use HDMI cables and asked about using the switching box that came with the RV. I had googled using the existing coaxial and everything I found said NO!. So as recommended by the folks here, I bought an electronic splitter for HDMI cables.
As far as I could determine the switching box I have can't carry HD signals. Everything in coax systems, every connector, switch, filter, and misc. component degrades or stops the HD signal.
My quest was to use one BluRay player for both sets and so I bought a 12vdc powered splitter(passive splitters don't work)box and 35 foot HDMI cable and installed them. The setup works great. The only thing I would or might, do differently is buying different MFG's of TV sets. Everything is Insignia (Best Buy) and my sneaky grandchildren mess with switching channels on both sets at the same time. One of the remotes will even switch the TV sets and the BluRay.
I have cable and antenna on the original system, F connectors, RG 58 cable and the splitter box. The BluRay is on the HDMI cable and electronic signal splitter.
The cost was minimal, under $50, the installation was pretty difficult mainly due to the age of our rig and in 25 years the RV industry has learned a lot about routing cables and other wiring and installation might be much easier.
I managed to find TV sets and a BlyRay player that had wall warts so everything except the splitter runs directly off the 12vdc system in the RV. I'm looking for one of those too.
