Standard vs Hi Def?

Just for the record: While DirecTV requires somewhat larger and more expensive dishes for receiving HD, Dish Network HD transmissions CAN be received on the smaller dome type dish antennas such as my Winegard RoadTrip Mission. You might run that idea through your mind if you already have such smaller dome antenna and if you want HD satellite TV and not the expense of a larger Dish on your RV.

As to your other question, I do not recall ever having a problem receiving HD transmission but still getting the SD channel.... won't say that cannot occur. Its pretty rare if it does.