Please don't give me sh#te for suggesting this, but, Minivans.
There is room for 4-5 adults and their luggage, and the things you acquire along the trip.
And the icebox.
You do need an icebox.....
One person drives the Van while others get to ride along in Vettes.Then you swap....
Most are V6, get great mileage and can cruise at 80-90 mph.
You can nap in the back if necessary