Hi @da_costa, I did do a quick google search but couldn't see any wings on said truck. Maybe you could pop an image on here? I suspect the purpose would be vortex generation to reduce pressure drag, but without seeing it I would hate to speculate. As for how much lift, if it was for vortex generation it wouldn't produce much, however, if you had a decent 3D model you could run a simulation to find out would be an interesting project I reckon.
Look forward to hearing back from you,