Why did you buy a charger? what powers the charger? Doesn't your van have an engine, an alternator and a 12V battery? Doesn't the van have a lighter socket? Then why didn't you simply buy a lighter plug?
You said that the speakers use a 12AC power supply. Inside the speaker, the full-wave-bridge rectifier converts the 12VAC to 15VDC.
If you feed 12.6VDC from the van's 12V battery then the rectifier reduces the 12.6V to 10.6VDC. The rectifier voltage drop depends on how loud the speakers are playing and is about 2V at full power.