Ok, you need a satellite dish. You can get a standard one like the one on your house, which you set up on a tripod and aim manually. Or you can get one of the motorized ones on the top of the RV, either automatic which locks onto the satellite when you are parked and tell it to, or 'in motion' which always keeps the satellite in line. Obviously, this list is from cheapest to most expensive.
Next you will require a directtv receiver. You can take the one out of your house, or rent a second one for the RV, or buy one.
And of course, you will need a TV, as well as cables to hook the dish to the receiver, and the receiver to the TV.
Keep in mind that getting 'local' channels on directtv is problematical. Depending on how often and how far you travel, you may be well served by signing up for an 'east coast' or a 'west coast' network feed (CBS, NBC, etc) for an additional monthly fee.