Good advice above but even with the stock light wiring you should still be able to charge the camper battery, it will just take a little longer. As the camper battery charges it will reduce the current flow through the charge wire and reduce the voltage dropped across that wire until the battery is fully charged. Unless you have another DC load in the camper causing a voltage drop across the charging wire. For example, running an absorption fridge on DC can keep enough current flowing through the charge path that the voltage drop from the charge wires never allows the battery to get the full charge. Make sure other DC loads in the camper are turned off and you should be able to charge the camper battery. But keep the wiring upgrade on your "to do" list.
