You can't get an HD picture with DirecTv and a portable sat. Only Dish offers HD programming with portables. If you want an HD picture with DTV then you'll have to use a manual setup dish. If you have two DTV receivers at home you can simply bring one with you for free.
We have Dish at home and I have a portable sat that we bring. I purchased an extra receiver that stays in the 5th wheel. It only costs $7.00 a month to use. I can stop and start at any month.
Dish has an RV program and caters to RV'ers better. DTV doesn't really have any RV programs. But you can just as easily use it. It just depends on what you are expecting of it.