You are comparing trailer to low load on the truck.
I your trailer enclosed or flat?
When I was pulling my 4500lb trailer empty with my flatbed, I was surprised with no recordable difference in mpg. I figured out part of that was that we do have 55mph limits for towing and part that dovetail trailer lower the air drag of boxed flatbed.
Another sample, had 3000 lb forklift on my flatbed, but with tall mast. That thing did lower my mpg quite a bit.
So my experience, the weight added inside will show different mpg in city driving, but not on highway.
When it comes to highway, all that matters is air drag aka frontal area.