The issue is that while it may be a trivial gain, that same photovoltaic material would generate more energy in a fixed installation. Also, as an installation on a truck, the weight of the system contributes to the energy needed to move the truck, somewhat negating the benefit.

So sure, have your electric truck. The trickle charging of any onboard solar system wouldn't even be noticeable though, and it's better to have the panels on grid helping drive the charging infrastructure. I saw someone guesstimate a theoretical peak of 25kw. My car charges at home at half of that, and even for my comparatively tiny car, that takes a long time to restore range, compared to it driving down the road. The truck might be able to get an extra 5 miles of range per hour of peak sunlight with 25kw system under realistic conditions, and that same material might be able to extract 40-50% more energy over time in a fixed installation.