Standard with a Lance camper installation is running a 8ga wire from the truck battery to the plug for the camper. 8ga is more than enough for charging the battery in the camper.
The heavy gauge wire used for the truck's battery/batteries is to enable a high current draw when the engine is being started. Very different demand than that of a 6-10 amp charging situation.
With a 40 foot total DC run the difference between 6ga and 8ga is only one tenth of a volt and going from 8ga to 4ga only increases the voltage by two tenths of a volt. Not going to be a measurable difference in charging time with 4ga versus 8ga in this situation.
