I don't have composite as truck sides but on my 12' landscape trailer I used ChoiceDeck for the floor. It is GREAT! Never have to worry about it rotting. I wouldn't think twice about using it on your truck. Especially since your F-250 is white, the gray boards would look nice.
Ford and Chevy Trucks
Scag, Exmark, Wright, Toro, Snapper
Echo, Stihl, Shindaiwa
Lesco application equipment
Having employees that enjoy what they are doing and care about quality is what makes happy customers.