What hes saying is with the truck running, the measured voltage it is supplying to camper.
Some of that charge from truck should be going to battery-which doesnt appear to be. (Along with converters charge power when plugged in or running generator)
My moneys on somthing preventing charge from reaching battery from the converter/charger. Whether wiring, connection or 'some' unit between.
Which is why I think its not the converter, thouhg I may be wrong, due to the 12v from truck seen by camper but not batteries? multimeter would prove.
It seems to me a quite a while ago someone else had simular problem and it was some how related to the on off disconnect switch- electronic fail and it acted as one way pass thru?