The issue is simple. All the electrical equipment in the towed vehicle is tied to the hot and negative side of the towed vehicle. Also with most motorhomes the charge line is connected to the chassis battery. So disconnecting the negative line won't work for several reasons. As soon as you connect the motorhome to the towed vehicle you now have multiple ground paths to the battery that bypass the main towed vehicle ground. However it isn't quite the same with the hot side. If you isolate the towed vehicle hot connection you won't have any way for the vehicle system to power up. The only power which would be present from the battery would be the direct connections from the components that require power to the battery which would vary depending on the braking system. The lights are usually powered by the coach. GM was technically correct but failed to account for the vehicle in actual use behind a motorhome.
