Most cases the 2nd alternator will be a waste of money (for the purchase, and for the parasitic drag). With a 'stock' trailer connection on the tow vehicle you can only charge so fast, and the extra output of the 2nd alternator will wasted in most cases.
A benefit to the 2nd alternator would be to quickly bring the truck's batteries back to full after a long drag of the glow plugs/grid or extended cranking in cold weather. I recall a few years back Ford had an electric heater for quickly warming the truck that required the dual alt setup.