Yes you can increase the wire size, and get a lot more amps. You can also change to a "Anderson Connector" - or forklift battery connector, and increase to say #4 wire and use that size connector to get up to 75 amps from the engine battery.
What I might recommend is something like a 3 wire locking type plug, in the 20 amp range, that is a reasonable cost at your local hardware store. Lets say you find a 14L20 plug and receptacle. You put the electrical box and plug on the truck, and wire the plug and put a cord on the camper, plugging it in as you load. Or put it near the storage compartment so you can get to it once loaded - if you ever forget to plug in as loading.
The 14L20 is rated at 20 amps, but is much more robust than a 20 amp rated camper wiring socket. Also because it has two brass terminals, and a ground and a silver terminal, you can wire each with a #10 wire. Use the Green and Silver as 12 volt grounds, and the two brass terminals as +12 wires. Then connect the +12 wires to a #8 wire going towards the battery. Have a forklift battery isolator such as Grainger part #6C017 (now $45) rated at a maximum of 100 amps and has silver allow contacts that are good for 100,000 on-off cycles (it will last well over 20 years in RV service). A 50 or 60 amp automatic re-set circuit breaker with 3/8" terminals will complete the circuit. Connect this to the battery +12 or to the alternator output directly. Run the #8 ground wire to the battery terminal for best results (lowest possible voltage loss).
Because the ends of the 14L20 plug will have 12 volts to them, and could spark against anything metal (powered by the camper battery) you should use a 14L20 cord end (the female side of a cord connector) to cover the plug end while the camper is off the truck, or otherwise unplugged from the truck. Even the bare wire can short to the truck bed while loading process, because both wires are not protected. A 60 amp automatic reset circuit breaker in the camper, near the battery is also required to protect the wiring from short circuits from the camper battery end.
Good luck with your project.
Fred.
