Solar panels: The roof of a trailer is about 450 square feet. In the northeastern U.S., you would average only 3.5 hours of full sun, so you'd get only a little over 13 kW per day.
So the solar generation function of a panel is no sun, no power, partial sun, no power, peak sun hours, full power.
Well, I guess that you're the expert.
So in an entire day, covering the entire roof of a trailer with solar panels would add a whopping 7 miles of range
Maybe that's enough to get you over a time zone or so so that you get an extra hour of "full sun"? ;)
After all, it's not like trailers are ever sitting idle during loading, unloading, or waiting for loads. Always moving, doncha know.