Thanks for all the info and different view points.
So far, in the short term, it worked great on our last trip. The truck camper battery is now grounded to the frame.
As far as boiling the electrolyte, my charge controller for our solar system on our previous motorhome had the "balance" mode that would ramp up and overcharge to balance the battery bank. I guess the argument against charging the truck camper with the alternator through the truck batteries is that it's possibly in a constant state of overcharge without any way to regulate it.
At idle, the truck camper/truck batteries are at 14.1v after driving for a couple hours. I don't have a remote meter to monitor the voltage while driving, and the dummy gauge in the truck is analog, not digital.