We went through the same with the kids, two girls and a boy spread out about four years apart each.
The oldest DD hit that age where she didn't want to camp so we suggested she bring along a friend. We bought a small cheap dome tent (7X7 I believe) and a full size air mattress. We called it the guest room. The next trip she wanted to bring two friends. Camping is fun again.
Our younger DD hits that age so she brings a friend. We bought another cheap tent and air mattress for them.
One trip we were in command of six giggly girls and I only owned two. We rented two sites for that one.
Went through the same with our son. The kids are all grown, youngest son is now 26, but I still have those two cheap tents and mattresses. I keep one set in the FW just in case. It's the guest room.