Your best bet for your first visit to DC is Cherry Hill RV Park in College Park, MD. It is the closest park to DC (there are no parks in DC). It is a little more expensive then some other parks that some will suggest, but pay the monies. Its worth it.
The park is clean, the staff is user friendly and you are only are a short bus ride away fro the METRO. The bus stops at the fron gate of the campground and takes you the the CollegePark Station. Or if you chose you can drive overt to the Greenbelt Station and catch it there.
The METRO is user friendly. You can preplan your trip here.. Or you can just talk to the Cherry Hill staff and they will help you.
I work across the street from Cherry Hill and I ride the METRO downtown to headquarters office all the time. PM me with any questions.
Mary, the world's best wife (1951-2009) R.I.P
Lizzy (the Boston)
Izzy, Pepper & BuddyP - Gone but not forgotten
2005 Itasca Suncruiser 35A
Go with Cherry Hill. If I read correctly, you have a scooter and two bikes. I would not suggest riding those to the nearest metro station anywhere in the DC area.
As for Acadia, we like Bar Harbor Campground, but, last time we were there they were still not taking credit cards. It is on the local trolley/bus route however. I would suggest that the National Park Campgrounds inside Acadia (No hookups) would be great, but I would be surprised if you could get a spot.
Travel safe and have a great trip.