While a roof top tent has its upsides (big comfy bed away from critters) you lose the ability to carry gear on a dedicated roof rack. When a full sized roof rack is used just for carrying gear — it can carry lots and lots and lots of gear. If a roof top tent is up there, only sleeping bags and pad will fit.
While a roof top tent might end up being your ultimate favored choice, my advice before buying one is try camping (sleeping) different ways first.
1. Pitching normal backpacking tent on empty roof rack with plywood floor (I did that often).
2. Placing a one man tent on top of a Camptime Roll-A-Cot cot (my favorite)
3. Sleeping on the ground in a tent. (Never been my favorite method).
4. Just sleeping inside the car (I did that extensively too)
Personally- while I thought about using a RTT, I’d never use one because I could never afford to lose the roof rack as a cargo carrier.