Intermodal trailers are fairly common but not the majority. Most trailers are box trailers, where the enclosed "container" part is integral with the trailer chassis. Intermodal trailers are used particularly in conjunction with ports and railyards.

Intermodal is actually a relatively small subset of trucking - probably behind generic dry van, reefer, and flatbed.

That being said, solar panels are great but what happens when they break down or get damaged. I don't think the savings are worth the hassle right now.