I think that he will probably get more fuel efficiency by doing without the trailer. Without much of a push at all. Especially if he uses some ratchet straps to fasten down the parachute as much as possible. In my experience trailers rapidly add to the fuel consumption, no matter how large or aerodynamic.
However, I am not a big fan of rooftop tents at all. I think having one would send me crazy.
In my opinion, you're far better off getting something like a Blackwolf Turbo tent, or OZtent RV5. Just as quick to set up, not on the roof of your car where you rock and sway. You dont need to climb up rickedy ladders that likely hurt your feet with no shoes on..... When you're not going camping for a few months, you cant get it off your roof without devising some sort of winch system or getting a few of your friends around.
Most of all though, if you decide to go for an afternoon drive you do not need to pack up your whole campsite.
