Others with more knowledge will likely chime in but here is my two cents... The wire gauge is critical but isn't as critical as some make it out to be. You can pull 30 amps of current from a 22 gauge wire, the question is how long that wire will last before it melts. Also, the voltage is also critical but is not going to define how well your camper charges off the alternator. It is the combination of these two elements that is critical.
Here's the thing, current flows where there is a difference in voltage. Your truck alternator might keep the truck at around 13.3 volts. If your camper battery is at say 12.6 volts you only have .7 volts of potential difference. That's just not going to flow a lot of power from one battery to the other.
This as others have said is where DC to DC chargers earn their keep. There is plenty of amperage that can be pulled from the truck but you need a special circuit that will pull this at a higher current and then use that current to create a higher voltage to the camper battery and thus force more current into it.
There is nothing wrong with upgrading the wiring from the truck, it is the first good step to take. But don't make the mistake and think that upgrading the wire is going to solve your charging woes. You need to invest in a DC to DC charger if you want to be able to get good results charging the camper battery from the truck. And, before the naysayers say "I have never needed a DC to DC charger, my camper charges just fine"... Yes, it will work and for some can work fine in many situations, but it won't be the most effective for situations where you need a lot of power for your camper.
