what are the more common culprits? Whatever it is it killed new fully charged batteries in about 5 hrs.

 

Alternator output is the first thing to check. Should be approximately 14.1 volts.

 

Is this while just sitting or running. Either way there is a big load on the batteries somewhere that can be narrowed down with a clamp on ammeter and pulling fuses one at a time.

 

Alternator is good or at least passed a bench test at the parts store.

This only happens while sitting and is not constant. Sometimes the batteries are dead and most times it starts fine.

 

A Bad GPR is a good guess

 

You should check for excessive battery drain or "parasitic loads".

These can anything from a malfunction load, GEM failing to Sleep, or others.

Its somewhat simple to identify. Using an ammeter. Make sure all electrical loads are off in the truck, doors closed, and the key is out of the ignition switch.

Disconnect one of the (-) battery cables from the battery, placing an ammeter in series between the battery post and cable clamp. The current draw reading should be less than 35 milliamps. Wait about 10 minutes for everything to sleep.

A reading higher than this would indicate excessive battery drain. Something is "on", allowing current to flow running down the battery. Vehicles today typically will draw less than .020 amps (20 milliamps) of current to maintain electronic memories and circuits.

If the battery is disconnected parasitic drains may temporarily increase. Circuits in the engine and body computers are activated and will run until internal timers run out. This reactivation period could be anywhere from a few seconds to almost 30 minutes. Whenever possible avoid disconnecting the battery while performing this test. It is possible to place one lead of the ammeter on the battery post and the other on the battery clamp, while at the same time lifting the battery clamp off the battery post. Its best to use Clamp-Style connections on the DVOM.

If the drain is active, begin removing one fuse at a time until the draw lowers to an acceptable range. This will be the circuit that has the draw. You may need a wiring diagram to further troubleshoot exactly what and where the offender is.