Temperature is a major factor as to what mileage you get. On colder days you're heating the inside of the van and the van is warming the hybrid battery, and maybe you've got the lights on, and maybe the kids are watching videos in the back - all of that comes out of your mileage. ANY sort of electrical use reduces the mileage, you've just got to keep that in mind. How you drive is part of it, but it's not necessarily the largest part.
Below is part of a reply I made to someone who was asking about what kind of mileage to expect.
I've got a 2018 also.
On days when no, or VERY little, climate control is needed I get the advertised 32 miles on electric.
Running the AC (front only) uses up about 5 of those miles if I'm by myself, closer to 7 if my wife is with me also, because she likes it cooler than I do.
I was driving it recently when the temps were in the 30s and the electric-only mileage dropped to just over 20 miles (or about 12 miles less than 'normal').
I regularly make a trip that is about 160 miles, about 135 of that being interstate, the rest is in town, often in heavy traffic. I usually set the cruise control on 74 on the interstate. I'm fairly light footed when starting from a traffic light (though not 'slow') and coast when I can when coming to a stop.
On no climate control days I consistently get just under 34 miles per gallon on that trip.
On low climate control days I get 32.5 mpg.
When it's hot outside and the AC is turned up more (still front only, we never have passengers in back) and is working harder it gets down around 31mpg.
In the cold temps reference above the mileage took a big drop down to 26.3 mpg.
Apparently the climate control works much harder to make heat that it does to make cool.
So, in 40 to 80 degree temps I expect to get 32 mpg. Below 40 I expect it to drop pretty fast into the 20-something mpg range. In the 90s I expect 31-ish.
You live in Chicago where it gets much colder than where I am (north Florida). Based on my experience you can expect your mileage (and electric range) to take a serious drop on the really cold days.
If you're going to have people in back and the climate control will be running back there also, my GUESS is that you can expect to drop about 2 mpg for that, and to loose 4 or 5 miles electric-only range.
The mileage on the PacHy seems to be much more sensitive to hot or cold temps than the Prius. My assumption is that much of this is due to the heating/cooling of the hybrid battery pack.