Output and input are not neccessarily proportional. In fact, generally the unit is designed for a specific input and varying that is not good for it.
With the alternator running, you should be a getting more than 12.7 volts. It should be well above 13 volts, perhaps well into 14 volts (the standard is 13.8v). If you are only getting 12.7 volts, you have one of two problems:
1. Your wiring is and connections from the battery to the amp are insufficiant. The result is a high resistance causing a drop in voltage. Your losing a few volts to heat. Make sure the wire is of appropriate size and all your connectors are real ones, not those stupid scotch-lock things. If you have it grounded at the battery, it needs to be the same gauge as the positive. If you have it grounded to the chassis somewhere else, it needs to be the same gauge AND it needs to be bonded to unpainted metal with a real ring terminal (not wraping the wire around the screw).
2. Your alternator is toast. If your meter the alternator output at the battery while the truck is running, it should show no less than 13.8 volts. Have it tested and NAPA or something and they can tell you if it is shot or not.