For your situation, Dish network would be a better option as you do not plan to have it installed at home. Dish tends to be more RV friendly as they have a pay as you go plan and you can purchase their receivers for under $100 where Directv prefers to lease their equipment.
We are avid sport fanatics and realize that many of our favorites teams and their respective sports coverage is simply not available OTA or on any of the free major networks. In many instances where we RV there is insufficient OTA signal to get much of anything anyway, with sat tv we get it all.
At one time Directv had an advantage with sports programming, recently that is no longer the case. In fact we are currently subscribed to Dish since the sports package we require is not available on Directv. I do know the SEC network is part of our channel line-up on Dish network.
We have switched back and forth from Directv to Dish numerous times and both are pretty much equal in programming and pricing. One reason we like Dish is using their online chat to easily make changes, many use this feature to enjoy local channels from where they are RVing.
Finally, there is the question of how you will acquire the 3 satellites. We set up our sat tv manually when RVing, takes but a few minutes as we have lots of experience. For newbies a portable automated dome is probably a better choice. You can get Dish HD but not Directv HD with current domes.