2015 Chevy Colorado won't charge our camper battery!

How smart is the tow/haul mode system? If it looks for a load and doesn’t see it, the tow/haul mode might not start. If this is the case, you might need a breakout box to check the tow/haul voltage. I don’t think you should ask this question directly, but instead ask the dealer or Chevrolet to explain what process tow/haul mode goes through when initiated.

Well it seems to me from the tests you have made already that the trouble isn’t with the camper or the batteries inside it, it is within the Colorado truck system. Just like we all thought. Even though the resistance readings you showed seemed okay I still suggest you compare the difference in voltage across the two batteries. It is important that you do that while the camper batteries are discharged and being charged by the alternator (engine running around 1,500 RPM) due to the current load on the wires. If the voltage difference between the two batteries is less than .3 volts then I would say the wiring is okay and something within the boost charging circuit has a problem.

To get to the point your complaint is that the truck won’t charge at 14.0 volts in tow mode as it should. Somehow I failed to understand that from the original post. I thought you didn’t want to use the tow mode or still found 14 volts to be insufficient.

The charge rate is set by the PCM. If the dealer were to replace the PCM or anything related to the charging system and towing harness the problem would likely still exist as the replacement parts would be the same. Perhaps a software update will be released one day to improve the charging rate while towing.

“Very low resistance was observed (0.006 ohm, I believe).”

You literally measured the resistance of the wiring. You could have one single strand of wiring, and the resistance would check out fine. But it wouldn’t be able to carry much of a load. That is the reason starter cables are very substantial, in size

It would be better to measure voltage drop

Google it, or maybe one of the other guys can explain it

Lets say that the difference in voltage between the two batteries is only .5 volts due to wire losses. A battery charged to 12.8 volts is going to have considerable more charge than the same battery at 12.3 volts. Charging the camper batteries up to 12.8 volts will get them up to a reasonable charge but as most us here know, at that rate it would take a very long time to charge up a large capacity battery bank that has been discharged.