[anijet]

The resistance of 40' of #12 copper wire is only .07 ohms, not much. Late model trucks usually have 140 amp (or larger) alternators. Truck battery, after a normal start, should be mostly charged within the first 20 minutes or so. Even a well optioned truck shouldn't use more than 60 amps running the AC, lights, entertainment, and whatever else is available (with a charged battery). Plenty left for charging the RV battery. I have departed a campsite with a less than 60% battery and it was better than 90% when arriving after a 7 hour drive.