Another vote for Pine Grove Furnace State Park. It's one of our local favs. They've redone the dump station so it's easier to get in and out. Just be aware they have NO full hookup sites. Electric only with water hydrants and dump station. The bathhouses are very nice. Great, quiet campground off the beaten path.
I've stayed at 5 of the 7 campgrounds in the immediate Gettysburg area. I like the KOA best as its tucked in the woods at the base of a hill, feels less like a parking lot. Some of the RV sites are on the small side. Granite Hill and Roundtop are both OK and both have wooded and more open areas. Road noise at Roundtop is an issue depending on where your site is. After these three, I'd rate Drummer Boy next and Gettysburg Campground last. Drummer Boy would be great except its literally parallel to a busy highway. Wooded almost throughout and huge, a good mile end-to-end. Gettysburg Campground gets a lot of love but its a flat parking lot, albeit with a large wooded portion. Sites are quite small there IMO.
All of them except the KOA have seasonals, but I saw nothing in the way of junked-up trashy sites at any of them. If quiet evenings matter, the KOA seems to attract the least number of party-hearty folks.
I'll add that we camp twice every October in Gettysburg for Halloween theme weekends. I could go on and on about amenities and activities, but if you just need a base for touring Gettysburg than any of them would be fine.
Drummer Boy knocked themselves out of the "top CG in Gettysburg" category at our last stay there, in 2007. The corporate machine took over - prices went up and quality of service (counter staff, cleanliness of facilities, maintenance of utilities, etc. ) went down.
We have camped at Drumer Boy, Artillery Ridge and Granite Hill. All good, but a little more traffic noise from the close proximity to the highway at Drummer Boy.
If you like state parks,I would suggest Pine Grove Furnace State Park. Electric hookups,new bath houses,2 lakes for swimming,halfway point on the Appalachian Trail.