When I upgraded the house battery system on my 1996 VW/Westfalia camper van I also wired an audio system upgrade.
However, I think the audio system running off the starter battery (namely the 650 watt 5 channel amp) is keeping the DC to DC charger from kicking on by keeping the voltage just a tad too low on the starting battery for the DC to DC to kick on. If I turn the stereo off the DC to DC charging works as expected.