You will need a slimline dish, tripod and about 100' of good coax with good connectors to start. Amazon is a good place to look. You can use a box you already have in your house. Just set it up and try it in the rv at home.
You will need a box that is compatible with multiswitch lnb on the dish. If not then you will need to install a swm set up in the rv and this can be complicated.
Bottom line is that if you already have Directv subscription, yyou do not need to set up a separate account and it is totally legal.
Direct TV has a good forum. If you do a search there for rv installations, you should find plenty of information.
Good luck and feel free to PM me if an questions
Steve