Since the Revell/Monogram Kenworth W900 is essentially a 1976-82, you can really use just about any trailer. The AMT trailers issued by Round2 are essentially 1970s vintage 40ft trailers and will look just fine behind your tractor. You might have trouble with the livestock trailer, NASCAR/moving van trailer, & lowboy trailer due to the fixed fifth wheel on the W900. If you really want a modern 53ft trailer, then you should look into the Moebius trailers.
AMT Fruehauf tank trailer:
AMT Fruehauf Day van (same as the reefer):
AMT Fruehauf PB-80 Flatbed with Revell/Monogram Peterbilt 359:
AMT Fruehauf Exterior Post van with AMT 1979 K100C Aerodyne:
I haven't built any of the Moebius trailers, but I know plenty of people who have, and they are nice kits.