If I had mobility issues and had a choice of bringing only one of an ATV or the scooter, I would bring the ATV. If the scooter is going to take up space 'inside' the vehicle, I'd say just leave it at home....I truthfully don't think you're going to find it that useful and will take my more time, energy and space that it's worth.
The area that we use is fairly large and you might want to park further away from others, or you might just not be able to get a parking spot close the center of all the activities.
The scooter is going to need to be recharged, the ATV just needs gas added.
The ATV will let you get out and explore the area where the scooter will mean just going around the RTR area.
The ground, while harder than concrete is still fairly rough with stones so the scooter is going to be in for a really bumpy, difficult ride. Karen, while fiercely independent (yes, you are Karen....
) still needed a push here and there in her wheechair and it's equipped with off-road wheels. Pushing a scooter is not so easy.
Since you can walk 50 yards or so at a time, you will be able to drive the ATV around and then walk to the camp circles, etc.
It would also let you carry a lawn chair much easier...packing lawn chairs on those scooters is difficult.
Just remember to drive the ATV slowly...same as the vehicles have to do...if you're creating a cloud of dust behind you, you're going TOO FAST, dammit.