Unfortunately, duct tape and cardboard (or better - score some free coroplast) is the best answer to your problems, if you're looking for "quick and cheap".
Because in an hour or so, off the top of my mind, you could easily fashion (in estimated order of impact):
- half tonneau
- partial grille block
- smooth wheel covers
- rear & partial front fender skirts
- fold back passenger mirror (if feasible)
And maybe you'll save yourself 5-10% on the cost of the trip.
If you have more than a few hours and lots of coroplast, an aeroshell style bed cap is your best bet.
Taping the cab/bed gap can't hurt, but it's not going to be high on the list.
Aside from that, slowing down is going to net you the biggest gains.
I'd slap the stuff on there and hit the road. No better way to excercise the "dorky" self-consciousness!