We love to camp. Our favorite places to go are Rocky Gap State Park in western MD, Tuckahoe State Park on the eastern shore, and for private campgrounds (b/c for what it is worth to you, there is a no alcohol policy at MD State park campgrounds now--this hasn't been a huge problem for us though) we like The Treehouse Camp at Maple Tree Campground. This is probably the tree house place you heard about. We've always tent camped there, but I have checked out the the tree houses and the cottages are really nice! The tree houses, however, are VERY bare bones. Personally I prefer a tent...
Rocky Gap is great because it is in the mountains, has a beautiful lake (which I believe you can swim and boat in) and nice hiking. Tuckahoe is great if you like to kayak or canoe--they have beautiful water trails. Both are very dog friendly and have nice, private sites.
The Treehouse camp is also really nice, but lacks that expansive feel you get at a state park. But it backs to the AT, and they make coffee in the office for purchase each morning :-)
And for gear purchasing, we like REI and LL Bean. Neither are "local" but we have had great experiences with both standing behind their products. REI Outlet online is also a great way to get deals--I always order online, have it shipped to the local store for free, and then return anything I don't want before I leave the store.
Good luck and happy camping!