RV video control center with hdmi input, coaxial output

I have done a lot of research for others on this and I have found some HDMI-Coax modulators (HDMI in, channel 3 or 4 out, just like the old NTSC- A/V in-3/4 out modulators for DVD's)

problem is they are costly

I know of nobody who makes one set up with a multi-switch for RV use. but

Google HDMI Modulator
For links to them.