IMO, Dish tries to compete on price and DirecTV tries to compete with more comprehensive offerings.
You can look up both on the Internet and see which channels they offer in each package and how much each package costs.
Depending on where you live, you may be able to go to a store or a few and see the programing that each offers and evaluate which fits your needs better.
They both use the same basic technology, but the programming and bells and whistles are different.
I think the new DirecTV Genie is supposed to service multiple TVs without requiring cabling to each TV (antenna dish is cabled to Genie box, and RF connects the box to the TVs without requiring any cables). Depending on your RV and available wiring, that might be of interest.