Campgrounds Baltimore Area

Another option is a little west of the city called Ramblin' Pines:
http://www.learntorv.com/2013/12/ramblin-pines-woodbine-md.html

I agree that Cherry Hill would be a drive that I would prefer not to make to see a Baltimore baseball game. Though, it could be good if you lined up a weekend that both Baltimore and the Nationals both play at home. You could see a game in both cities.

The KOA is probably your best option.
Cherry Hill is doable but a little too far away.
Patapsco State Park is your closest Option, If you like state parks.

Morris Meadows and Bar Harbor are options but they are north of the city and out of the way if you are coming from the south.

KOA

Patapsco State Park

Cherry Hill RV Park

Capitol Koa is closer in to Baltimore and a very nice cg in terms of cleanliness and helpful management.

Cherry Hill is the place pool and great place to be in that area