Need Help! Aero Drag vs Larger Engine.

[CANCarbGuy]

Hey guys, I have a problem and am hoping your practical experience will help. I have just graduated University and to celebrate, my friends and I are heading on a 4 day camping trip. Roughly 6-7 hour trip one way, 98% all open highway. Money is tight to say the least and literally every dollar counts, and as such, we are having a disagreement as to which vehicle would get the better MPG there and back.
My car is a 1995 Oldsmobile 98 sedan (3.8 series ll V6, 4 speed auto,) I do all regular maintenance with synthetic fluids and have a few low buck eco-mods (over-inflated tires, removed extra weight, etc). I recently changed the O2 sensor and spark plugs if it helps. On the highway, driving conservatively (normally I guess) I get about 31-32 mpg (traffic and weather dependent).
My friend's vehicle is a 1993 Toyota Corolla wagon (1.8 I4, 4 speed auto). I'm not sure of her maintenance but it appears to be in decent condition and she says 34-35 is normal for extended highway driving (again traffic and weather dependent).
(Please note that these figures are with only the driver and maybe 1 passenger, no luggage).

Now for the big issue. We've mock-loaded each vehicle, and provided the front passenger has a backpack between their legs, we can just squeeze everything (tent, cooler, equipment, duffle bags/suitcases, toiletry items) into the trunk of my Oldsmobile. Due to the size of the Corolla wagon, at least 2 suitcases (or the equivalent of) will have to be bungee-corded to the roof rack of the vehicle for everything to fit. I am extremely worried that the added drag from whatever we load onto the roof rack will stress the engine and inhibit our mileage. Just the 5 of us alone probably weigh 550+ lb with clothes on (2 friends are pretty small), plus another (I'm estimating here) 250-350 lb of stuff. I admit that I've never had my vehicle fully loaded before, but I am under the assumption that with less drag and far more torque (literally double at 230 vs the Toyota's 115), the V6 won't have to work nearly as hard and save fuel as a result. Having said all of this, I am coming to you guys to get your perspective. Does anybody know how much the drag will effect the wagon? Has anybody tracked the mileage of a loaded 3.8 series ll V6 vehicle? Any and all info, thoughts, or speculation is appreciated. Experience wise, I'm a little out of my zone on this one.