Dish Network offers a no-contract plan for RVers. You can start and stop as needed on a monthly basis with no start up fees except for a $99 one time HD fee. You will need one of their receivers and a satellite dish. Go to www.winegard.com
and check out the details.
We have DirecTV at home and will be bringing our DVR and bedroom receivers with us. I just ordered a DirecTV HD 3 LNB dish on EBay for about $135 plus shipping. As long as you are a paying DirecTV account holder you can use your service anyplace it is available at no additional fees.