charging of 2 camper batteries with truck alternator?

As others have said, the hot wire in the trailer wiring harness is intended to allow the TV keep the trailer batteries "topped off" during transit. As you have assumed, the wiring is not really heaving enough to allow enough amps to quickly change a depleted battery. An easy fix is to move the truck up near the trailer, and use a set of jumper cables from the truck battery to the trailer battery(ies). Yes, the truck has to be running, and as said, above idle is better to get full output from the alternator.