My gramps had told me a long time ago that after buying a new vehicle the
break-in should be as:

For the first 500 miles don't take it over 60 or drive to long at one
speed...Vary it around a bit...After 500 miles, change the oil &
filter....after 10K, change the tranny fluid & filter....

Now I know trucks/cars have changed a LOT since my Gramps' days and it may
not be necessary any more, but it's engrained in my head and I still follow
it.  I have never had a tranny problem before in any vehicle and never had
internal engine problems or oil leaks before (including a 94 Saturn which
has known oil consumption problems...)  I even got 250K out of a 93 t-bird
with only maintenance when I sold it...But again, there are other people
here that will probably tell you it is unnecessary to do what I do.  As for
towing a trailer, I'd wait until after the 500 miles....(just an opinion
everybody else, so don't flame me too bad, ok?  :)

IYM....

"Break In" is all described in your owner's manual...  Crack it open
and see what it says..