Hi medavis,
Some of them them are Walthers, Some trucks N' Stuff, A-LINE, Athearn Herpa/Promotex, and most of them custom detailed and decaled for the "big" fright companies in the US.
I get most of the trailers and cabs undecorated from Promotex and A-Line and use home made decals to finish em' off.
I have been into trucks since my grandma bought me my first Hotwheels cement truck when I was 6 months old.
If you want to get some nicly detailed trucks, get them from A-line:
www.ppw-aline.com and Promotex:
www.promotex.ca They have some nice stuff. Also look on e-bay and you can find some nice deals on trucks. Just go there and search HO Scale. Then type in Truck or Trailer and you get lots of stuff that pops up.