While trying to wrap my brain around this concept I stumbled onto a better picture of the truck from Charles Phoenix's website.
Apparently the Chicken Dinner Candy Bar contained 0% chicken and was promoted more as a filling meal than as a snack. Since they don't seem to be around anymore we can assume their marketing failed on all fronts.
Further investigation revealed that the company created not one, but a fleet of Chicken Dinner Candy Trucks to spread the word. This picture from the "Kitschy Kitschy Coo" website confirms no less than nine of the trucks existed at one point.
While this proud poultry parade may have disappeared from the highways forever, my own personal travels brought about the discovery of a similar chicken themed vehicle. This picture was taken near Paducah, KY.
This modified El Caballero may or may not have promoted a chicken related product or service. I have no details about it and it will most likely remain shrouded in mystery forever. And that's probably ok.