I just finished some HDMI runs in my house and cables up to 50' are readily available and reasonably priced through Amazon. After some research though I found that depending on the cable there can be some lag in anything over about 25'. Longer runs may work and are doable they may work fine however they also can require other equipment. Fortunately for me I was able to keep my runs at 25'. I had previously used several different brands of HDMI cable for connecting equipment. For the most part they all worked fine. What I finally settled on was Blue Rigger cables through Amazon. The shorter cables I had tested from them were well made and worked well. They were not the cheapest cables however they were very reasonably priced. They were also CL3 rated for in wall installation and had category 2 certification. I bought one to begin with and did a test run and found that it worked well. Went back and bought more. I actually ran a couple of extra cables to the main television just in case.
These are the cables I used
Blue Rigger 25' HDMINow personally I am not sure I would go to all the trouble to do the same in my RV. I think (know) I would just opt to buy a small blu ray player while they are on sale fairly cheaply during Christmas. That way you can have the option of different movies playing at the same time.
Technically there is a way to run HDMI over coax however it will require an HDMI over coax receiver. Probably not the most cost effective method but is doable.