Swags by all means have there uses. But a 3 week trip... Yea no thanks.
You have no dress room, so if it's raining you gotta get dressed laying down..
You'll dressing from the back of the car, another pita..
Definitely take the tent.
The 10-15 mins put uo and tear down is nothing, you just have to pack smart.
A small bag of clothes each and a 4th for dirty clothes. So when you come past a laundry it's ready to go.
Not sure of your setup, we live from the back of our car, we have a small table for gas cooker to sit on, fridge in the back and pantry is in the draw system.
So once our tent is up, clothes into the tent and once beds are uo were enjoying a coldie..
Imo, tent