You have a parasitic electrical drain (unless the battery is a piece of junk , which is relatively common these days) Does the battery stay fully charged when disconnected from the vehicle?
You're going to have to test each circuit but the voltage regulator is a likely suspect so start there.
1. Attach a multi-meter set to Amps with the red wire in the high amp jack.
2. Set the dial to DC amps.
3. Disconnect the negative battery cable from the battery and connect the test leads of your meter to the battery terminal and the negative cable.
4. The meter should read 0.00 amps
5. If your meter shows any amp reading, you have a parasitic draw.
6. Disconnect the BATT wire on the regulator while watching your multi meter.
7. If the reading drops to zero, the cutout relay contacts on the regulator are likely stuck together causing a constant draw.
If the voltage regulator is not the problem, continue testing the other circuits (ignition, lights, heater, horn, etc) on the truck in a similar manner.
1. Disconnect main wire on the back of the ignition switch.
2. If the reading drops to zero, replace your switch.
Do the same thing with the fuses on the firewall, etc.
