OK, let's get real here folks!
If you want DirecTV and HD then the only kind of antenna that you can use is the same kind of antenna mounted on your house. There are no portable automatic antennas (unless you consider a permanently roof mounted antenna portable) that will give you DirecTV HD. No Domes!
It takes a bit of practice but you can get a home style open face antenna mounted on a tripod, run some coax to the RV and point the antenna yourself at each new location. This is your ONLY non-roof mounted option. The roof mounted option is a Winegard Trav'ler and is pretty expensive (but AWESOME)!