Semi trucks and trailers are two different components. Most trailers have a large flat area, but these can get swapped often. And even within the same company you won’t always have the solar trailer hooked up to the electric semi. The swappable nature of the load is part of the appeal of semis.

I guess what I’m saying is, even if you had a solar trailer, you would lose a lot of the load flexibility of using a semi truck. And you wouldn’t want to plan a load where you needed the solar power. So either way, you’re planning for charging at fixed intervals. Solar might help a little here, but not very much.