The problem is, you're trying to figure mpg's over 185 miles.
There are too many inconsistencies from one station to another, filling procedure, angle of parking lot, driving style, terrain, traffic, etc to make any conclusive mpg
claims based on that short of a test.
test it over at least 3 tank fulls, same exact driving conditions, road, filling station, and filling procedure (fill it until you see liquid diesel sitting in the neck). Then test without the weight.
I'm sure you'll find that less weight yields you better mpgs, and the higher figure you saw with the trailer was a temporary exaggeration for 1 or more of many, many possible reasons.
moving this to MPG
for you where you'll get more hits.