I have a King Dome in-motion sat on the roof of my 370XL. It's circa 2008 and so it doesn't have the latest/greatest DirecTV transponders to pick up the new HD satellites, but otherwise I'm VERY happy with it.
There have been a couple times that it get's confused after long periods of no power, but I just use the reset procedure in the manual and life is good.
I've never tried driving with it on, but I can tell you that it's always locked into the signal by the time we park / turn on the TV.
A pic of how it looks mounted: