This guy did tests in a different EV truck https://www.youtube.com/watch?v=UmKf8smvGsA

The results were:

1. Adding weight to the bed, if it doesn't affect aerodynamics, doesn't affect highway range much. For stop-and-go traffic I assume the range would get worse, but he didn't test it.

2. Adding a big air pusher to an otherwise empty trailer murders the range

3. Adding an aerodynamic car or truck on a trailer is better for your range than the air pusher

I do wish it was a hybrid. Maybe small companies don't have the knowledge built up to make a good hybrid drivetrain but hell, Edison is going for it. They're planning to build logging trucks with a diesel generator under the hood as prime mover for a series plug-in hybrid drivetrain. It looks very practical and their initial tests show it tows great. (Since that's their entire selling point, they'd have to fold if it couldn't haul logs)