leitmotif,tclederman and red58 have given good advice. Use an "Amp" meter rated over 40 amps and hook it in line with the Armature Connector on the genny. Start the truck and see what current you are drawing. Turn on items like headlights one at a time When all including the heater motor and wipers are on you should have less than 40 amps. If you have more you need to determine which one is pulling the most. If the regulator is working a volt meter at the battery will read 14.5 volts DC when it is charging. Do this at a fast idle so that the genny can do it's work. One thing at a time will let you know what is going on with each load that you have. I have seen heater motors that run but draw so much current that they will drain the system rather quickly. Wiper motors can draw a lot as well. Halogen head lights can pull too much as well. A bad coil or no ballast resistor will use a lot of current. So do this 1 item at a time. And let us know what you find.
