Yes, you can go for just two days and yes there is a charge for camping under the wing. Camping is in a separate area with nice facilities and is worth it (in my opinion). Three night minimum charge even if you only plan on staying one night.

Unless you're in a homebuilt, finding a spot can be tricky. Sometimes it's full and they turn away Cessna/Piper/Beech traffic. Showplanes (homebuilt, warbird, etc.) always have a spot.