There are automated systems that can locate a Directv SD signal for around $500, but for HD it is closer to $1800 installed on rig's roof.
When we installed a couple of flatpanel HDTVs in our rig a few years back we dumped our 18" round Directv SD dish and picked up a used HD sat dish that matched the dish we have installed at home. Just bring HD receivers from bedrooms at home.
Mounted the Directv HD dish from Ebay on the same home-made stand and found the process no more difficult than aiming the 18" SD Directv dish as we still just lock in sat 101 with a 90% or better signal utilizing nothing more than a keyring compass.
Just make certain the HD dish is set to receiver indicated values and mast is level. I guess we have lots of practice as we have been doing it for a number of years with the SD dish and it takes maybe 10 minutes tops with a clear shot of the southern sky.
We camp too often where we might need a 100' (or more) of coax to lock on a signal through the trees so a rooftop dish wouldn't serve much purpose. Even use satellite when an RV park provides cable tv since channel selection is sparse and cable signal is weak providing substandard picture by comparison.