The charge voltage the tow vehicle battery needs is measured at its battery. The second battery is not a load so the voltage is not increased just because it is there. Operating in tow-haul or turning the headlights on will increase the output voltage. You need a larger gauge wire to carry that voltage. You would still have to measure output voltage to see if the increase is enough.
I loathe the EPM system in our GMC Acadia. It doesn't ever fully charge the battery it leaves the battery at low voltages, 12.4, and lets is sit there and sulfate, reducing its capacity and preventing it to ever fully charge again. The battery goes into a death spiral.
