All that is needed for the short run from the engine compartment to the receptacle in the bed of the truck is 8ga wiring for the power and ground lines. You can use thicker gauge but it is not going to make a measureable difference in charge times.
You may also have a failing alterntor with your truck. Easy to get it checked out in terms of output. Second component that can cause problems is the voltage regulator. Third possibility is a failing battery in the camper or in the truck. A bad cell creates added resistance in the circuit.
