The alternator simply holds the voltage at the level determined by the computer. And yes the program does not always hold a high voltage as it did in the old days.
No. Connecting an additional load does not alter the alternator charging profile. That second battery is just another load same as turning on the headlights or air conditioner.
You can connect that battery at 12.3 volts very close to the start battery and you will get a decent charge going. The nature of lead-acid will have the charge level tapering off quickly as the alternator just holds the 13.x volts.
Now connect with the OEM #14 wire 25' back to the trailer and the voltage will drop causing the tapering off to be even worse.
Enter the DC/DC charger and that voltage drop back to the battery can sag down to 11 volts and yet you will still have full 14.5 volts on the trailer battery. Still takes 2 to 4+ hours for a charge.
