Jeff,
Placing an antenna between two 'layers' of metal, the cab
and the trailer, really isn't the 'best' idea in the world.
Getting it as 'in the clear' as possible is a very good idea.
With that in mind, where ~can~ you mount an HF antenna on your
truck? Also keeping in mind that due to the nature of a tractor
and trailer, there's really not going to be a good place, but
several that are 'less bad' than others, both electrically and
probably more important, mechanically.
Not the most informative answer, but I'm sure you know your
truck much better than I do...
'Doc