What's best and cheapest cable network for RVing

It is totally legal to use your sisters receiver for dish. Asl long as she says she is using it and authorizes it for use in an rv. I use my home receivers in my rv and directv knows it. They have it listed as rv receiver on my account. They don't care who's rv it is in.
As far as your question, it is common to have to reauthorize receivers that have not been turned on for a while. With directv, this can be done online via logging on to my account.
Steve