Emanison

I picked up a 2007 Silverado 3500 8.1l 6-speed Allison a few weeks back with 100k miles. I’ve been doing a lot of towing with a small load of around loaded 6,600lb 8.5x20 cargo trailer. I’m wondering if someone could tell me if the downshift from 5th to 4th is the transmission slipping or is it the torque converter locking after the shift? 6th to 5th shifts are very smooth.