Assuming I am understanding that you will be using this at home and while out in the RV then you want to have DirecTV install everything at your home. With a 2 year contract it's all free. Then ask the installer for an RV kit which should also be free. This will be a tripod mounted antenna exactly like your home antenna which is required to receive HD. You may have to get a better tripod but at least you will have all the equipment you need. The Trav'ler is mentioned above as "the only way to get HD" and that's obviously not correct. If you want to spend $2000 for a roof mounted antenna then the Trav'ler or the RF Mogul antennas will provide the solution.
When you travel you simply take a receiver from the house to the RV and away you go. If you don't think you want to take a Genie with you from the house you can have them install a Genie plus another receiver (not a client) like the H24, H25 or a DVR like the HR24.