I am not familiar with Dish but am very familiar with DirecTV. You must have a two way connection between the receiver and the antenna. The return signal to the antenna changes the channels which is why you see an error. Several things can block that return such as a non two way splitter or as is very common running through the cable TV amp. You must make sure that you bypass the amp with your coax.
I always get a kick out of the RG-6 comments which you can ignore. They are posted by people who have never looked up the line losses between different cables and are meaningless in the distance run in an RV. There is NO standard for RG-6 or any RG. I could take any old cheap coax, stamp RG-6 all over it and legally sell it. RG was an old term used in the Military and meant Radio Grade. That went out many years ago and was replaced by Mil-Spec. I work with this every day. Believe me a good quality RG-58 will easily outperform a poor quality RG-6 every time. People who say that they got a signal in an RV with RG-6 but not with RG-58 just means they put the connectors on right when they installed the RG-6 and probably had a bad one on the RG-58.