I don't really think it is the 1000 lb load, it is the startup and stop & go, probably a good number of stop lights and/or 4-way stops with some amount of expediency getting back up to speed.
I think I get around 15 - 16 around town with a heavy utility trailer loaded with 500-1000 lbs of cargo ... or like 1 yard of top soil.
My van easily sits around with an extra few hundred pounds of panels, and routine tools; and a square roof bar. Had to make a trip out towards the mountains today, to check on a property, do some quick maintenance, take some photos & measurements for some upcoming weatherization. Chucked a few extra things in the van, a few extra tool cases, a ladder or two, lawn mower, weed whacker, leaf blower, ... I did take some bags of mortar out, figured I didn't need to haul those. LOL.
Outside of rush hour, and I wasn't in a rush ... I know the route well. Rural highways with a few traffic lights and villages / towns, a couple steep hills to climb, ...
Just got back:
It blipped 31.5 a couple times, slowing down mid way in the return, but I knew that wasn't going to be attainable. The computer goes something like 29.7 30.1 30.3 30.7 31.1 31.5 ( with the lit/100km conversion our vans do ) and this new best trip translates to 7.5 lit / 100 km. Which is about what my old 1.8 liter VW Polo / Golf / ... engine did back in the day.
Pretty solid.
Anyway, I hope there's some constant speed interstate or rural highway trips in OPs future, so he can experience some better mpg numbers. Go for a little drive into the country side on a Sunday morning
Also make sure your tire inflation is good all the way around, ... I know I need to check mine.