Found some interesting information on newer gm trucks. 2005-2008 gm half tons. Silverados tahoes escalades denalis. The alternator has a few modes of charging. A economy gas saving mode and a higher output voltage under tow modes. I monitored my voltage and found it drops into 12s and 13s after running for a minute or two. There's a voltage control module clamped around the negative post on my truck. I found a gm write up on how to bypass the voltage change. It's just turning the tow haul mode on. Here's the section from the forum I found it on.
Some owners may comment that when towing or hauling a camper/trailer that their auxiliary battery for the camper/trailer battery will not stay charged. In most cases, this concern is blamed on the new Regulated Voltage Control (RVC) system. While the new RVC system does reduce the generators targeted output voltage to 12.6-13.1 volts when in "Fuel Economy Mode", this feature is bypassed if the tow/haul feature is enabled. With the tow/haul feature enabled the system will stay in "Charge Mode" and the targeted generator output voltage will be 13.9-15.5 volts, depending on the battery state of charge and estimated battery temperature.
To correct this concern while towing or hauling a camper/trailer use the Tow/Haul feature.
Note: A second way to increase the generators targeted output voltage to 13.9-14.5 volt, is to turn on the headlights.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.