You will need to get your test device and check the following:
https://www.ebay.com/itm/29142802243...0aAuieEALw_wcB
When looking at the diagram on the left, (Truck bumper) the plug labeled 12V+ auxiliary must be hot when the ignition is on. This is probably your problem, batteries are not charging when on the road.
Your owner's manual will list the fuse that must be installed.