Thanks Time2roll and GaryM, Yes I do get that a DC to DC converter should do what I need. But those seem to be hundreds of dollars. So before I consider putting down that kind of money I really want to understand why a factory setup is not longer applicable. I've read your and the other posts multiple times and something still eludes me. Please tell me if I have it correct below.
I think what I'm gathering is that the alternator will see the sag as I plug the trailer in. But it doesn't see the additional battery as a battery. So all the electronics see is an additional load as the starter battery and the trailer battery try to equalize to each other. So the alternator only bumps up the output to get the whole system back up to 13.x volts and not into any kind of useable charging mode. Or put another way, even though the trailer battery is needing a strong charge, the alternator is only putting out enough to float charge?
Thanks
