Since your roof mounted satellite dome can do either satellite service provider and if you don't currently have it set up at your home, you could choose the service that suits your requirements best. If you already have service at home simply add the receiver to your account.
Although both offer pretty much the same exact channels with a few exceptions, primarily sports programming, they do differ a bit from an RVer's perspective. We have had both over the many years and change from one to another mainly for better pricing and upgraded equipment.
I know I want HD and since your current rooftop equipment can deliver this content via Dish Network, this is the route I would go. Pay no heed to the receiver in your bedroom, you can easily purchase and own Dish VIP receivers that are intended for RV use.
Not to slight Directv since we have always enjoyed their service as well, they prefer to lease you the equipment tying you into a long term commitment. If you are looking to add service at home it's not a big deal since Dish does pretty much the same thing.
The kicker is that if you only want monthly service a few times a year for RV use, Dish has such a package available, you can choose when and where to use it, they even accommodate address changes if you prefer viewing local channels in the area you are RVing.
We currently have Dish network at home and thus use it while RVing, we have always manually set up our own HD sat dish when we arrive to our destination. If you go Directv and want HD, for automated acquisition you need a roof mounted Winegard Traveler for nearly $2K, manually pointed (dish and tripod) is under $200.
