I have taken our son twice by myself the past few months. It was tough setting up and tearing down. I ended up putting him in a cheap stroller and walking him around while I connected/disconnected everything.
Camping with a 3 month old really wasn't all that hard. We just made sure we weren't too far off the beaten path with our campgrounds. We took a Fisher Price chair for him to sleep in. Worked out well.
Another thing we have learned is to start travel at 7:30PM. We leave right at his bedtime. He ends up sleeping the whole way and I leave him in the truck while setting up. I also put the monitor in the truck so I keep tabs on him while working on the camper. Once set up and the air is on, I try to sneak him in there without waking him up. I'm about 50/50 with that task.
It was way easier at 3 months than it is now at 15 months. At least keeping up with him anyway. It has been more fun each time we go, with him beginning to do more.
Congrats with the twins!!!