Does anyone know how rigid the check in process is? I'll be traveling from DC and probably won't be able to check in until the evening and they list the check in time as 1 pm. Is this just a mail drop check in or is there an actual Ranger/kiosk?

There is no formal check-in, the spot is yours after 1pm. There is no one watching over things that I could see. My scout troop didn't show up till almost 7pm just last week. Just make sure you're using the park map to find the campsite, it's hard to see from over the canal.

There is a drop box located at the campground. There is sometimes staff across the street from the campsite, but usually not. Reserve a spot through their website. I believe it's ReserveAmerica.org. look at a site map to book a site. The sites are on the opposite side of the canal from the road with 1 pedestrian bridge at the center of the campground to cross. The sites closest to the bridge get reserved quickest.