I agree with Scott, aerodynamically, you're going to make out, of course the
extra weigh you need to pull up things is going to hurt you... but look at
trains, aerodynamically they overcome drag to a HUGE degree over the
equivalent cargo on trucks. Quite a huge energy savings since each car is
drafting behind the previous. Indeed Brady and I have dreamt many times
about one van flat towing the other on a long trips(say SDM next year?)
since you would benefit quite a bit in drag and thus fuel savings.. imagine
if the whole mexico caravan could effectively draft each other.. much like
birds flying in formation, the lead van(say something with big power) would
do most of the work of overcoming drag, and everyone behind would benefit.
It'd be cool if not kind of dangerous to see what kind of benefit could
actually be realized.

Jim Akiba