[Carl n Susan]

If the battery is at 11.9 (aka dead) then it likely isn't being charged. I assume this measurement was taken with power attached. A definite sign the converter has either failed or the reverse polarity fuses are blown due to hooking up the "less than 6 months old battery". Check the converter's output - it should be around 13.6V. Look at the four fuses and ensure they are ok.