Just to be clear:
You are swinging north of DC and visiting Antietam and Gettysburg, right?
Do not skip Antietam/Sharpsburg. It's not as famous as Gettysburg, but it was nearly as critical to the war and it's a much better preserved battlefield (though Gettysburg is very good). It's perhaps the best preserved battlefield of all; and along with Gettysburg and Marye's Heights at Frederiksburg, one of the most tactically accessible of tourist of all the major battlefields. I spent several years living in DC and traveling to the major VA/MD theater battlefields; these were the best.
The civil war medical museum in Frederick, MD is also worth a look-see. It's small, but good.
To be on -topic, yes, what ArtP said, it's your temporary domicile while taking a day shuttle so how you store it inside as long as the door is locked is good. For a fun diversion of interst in DC if you can manage it, have a blue-plate lunch in the cafeteria of the Masonic Temple on 16th St. well north of downtown. An impressive, cheap little experience.