The card would come from DirecTV for $20 with a new subscription. It's possible the receiver can no longer be activated but DTV should be able to get you that information. You will have to agree to a new 2 year contract, you will not be able to receive ANY DirecTV HD programming and the antenna may or may not currently setup for DirecTV (but guessing that if you have a DTV receiver it could be).
If you are a part time RVer there may be other options that would serve you better. If you will be a full timer, there are antennas that would serve you better.