Originally Posted by Big Red and 30WR
Mine will charge the battery without using tow haul mode
On late model GM vehicles, the energy management system will drop the output voltage of the generator to around 12.9V after certain parameters are met such as set speed, battery fully charged, headlamps off, wipers off, etc. And while that can charge the trailer's battery it is not ideal.
The GM's computer does not monitor the trailers battery or voltage, so if traveling with 12V fridge running, you could discharge the battery while driving; I know I did it. That is when I called a GM engineer in MI and asked why my 2013 Silverado did not keep my batteries in the trailer charge. He noted to me to keep the headlamps on, or go into tow/haul mode. (And he also stated it is in the darn owner's manual so read the book.
In tow/haul mode or when you turn on the headlamps, the computer regulates the voltage output to around 13.5 (something like that, I can always get the exact spec, just not this late at night-especially when a two-hour time difference between here and MI). Whatever the voltage is, it is enough to send higher voltage to the trailer batteries, and after that info was received, I have always run with the headlamps On (which also helps other vehicles to see the trailer since the lights are all on), and I have never had a dead set of trailer batteries after 8-16 hours driving with the fridge running on 12V.
I'll see if I can get him at work tomorrow and get him to email me some of the specs for how the system works.