If you fit solar panels the size of the room of the X would give you, what, 200w, maybe 300w? It's resonable, depends on what you do. 14amps for 200w @ 14v. After a full day of sun he could pretty much charge a single battery from dead or close, but of course, the sun isn't as strong in the morning and evening than at mid-day,and you would have to worry about the angle and shade. For a camping trip and light use, or if you have a camper and just need the battery for the water pump and lights at night like my parents old 19ft trailer, it would work very well.
It all comes down to why and what he want to use it for, but on a vehicle that moves, unless you plan to park your truck for a week and use the battery regularly, it would be hard to validate it.
Unless you want to get into a cool system like the Prius and old Mazda 929 which have solar panels to keep the car cool on hot sunny days....And the 929 the solar panel was small, just in the sunroof, and that's old 80's technology too.