Pretty sure your PD 4000 is a converter meaning it only charges the battery when plugged into shore power. The voltage on the 12v pin is what is applied to the camper batteries. Using the truck to charge the batteries works fine, provided you don't have any other significant 12v loads in the camper, like running the fridge on 12v. The current driving the other loads causes a voltage to be dropped across the charging wire, limiting the voltage applied to the camper batteries. Hence the recommendation for a heavy gauge charge wire in the truck. The larger the wire the less voltage will be dropped at the same current. Provided you don't have other heavy loads, the existing wiring will work fine. Initially you'll have higher current going into the camper batteries and that will cause a voltage drop across the lead meaning you won't have the full 14.13 at the battery terminal, but as the battery takes a charge the charge current will drop causing the voltage dropped by the charging wire to decrease, bringing the battery voltage up.
There's nothing wrong with charging the camper batteries off the truck's alternator, but be advised that, at idle, your alternator isn't going to be putting out the full 150 amps. This makes it difficult to give you an estimate how long it would take.
++ on the solar suggestion.
