We do it every time we are out of our local spot beam. Call Dish, tell them you are in your RV and want to change your service address (not your billing address). They will ask for the local address and reprogram you to the nearest major city with local network channels. About a 5 minute phone call and another few minutes for the change to take place and the new channels to show up on your program guide.
Sometimes a zip code is all the want. But sometimes they want an actual street address. If you do not know the actual address, just tell them it is 100 "nearest local highway", city, state.