I routinely get 20-21 with my '02 Silverado 5.3v8 4wd, I get 17-18 towing my UXV450i on a home made trailer
On the other hand, My Tacoma 4.0v6 4wd only gets 14-15 and I haven't even tried towing yet
right now, my Silverado is in shop getting new rocker panels, service & annual state inspection, which reminds me to put another bug in my mechanic's ear, I didn't learn till it was too late, the shop I use is a little bit slow when it comes to body work