There is no reason a box can not be built to encode or re-encode, or simply copy the code for HDTV to a psuedo TV transmitter using either QAM or ATST encoding and send it down coax to a 2nd TV.. In fact there are some boxes on the market that do just that.
Now.. IF I were designing a modern Sat Receiver, epically one being marketed to RVers... I would include a "Card" in the box that did just that... alas... I am not a Sat Receiver designer.. But who knows.. I mean I'm not the smartest engineer in the country by a long shot. Perhaps one at least as good as me works for Dish.