I don't think you are describing what you are trying to do clearly.
It sounds like you have an RF cable TV wire that you want to connect to a rarely used TV with HDMI inputs in your motorhome? The cable TV output from your wall requires a tuner (cable box or QAM tuner in your TV). You can connect that coax RF cable with an F connector to the antenna input on the TV, set the TV tuner to QAM, and run a scan to see if there are any unencryted channels. If you don't get any then you will need a cable box to feed the TV. You could use a distribution amp to split the HDMI output of a cable box and run one of the outputs to that TV. You would need a way to control the box so an IR extender would also be needed...