The easiest solution to this problem is to fit an inverter, drops 24-12,
look at the boat magazines and catalogues. Only problem that I have found
with this is the output quality is not stunning (can cause problems with
electronic ignition), however I run my stereo off it with no problems.
Mine puts out 30A at 12V peak. Get a good quality unit otherwise you'll
probably regret it.
The better solution would be to fit a second, 12V alternator, linked to
it's own battery, this is what I intend to do but it's a bit of a hassle.
Regards,
Ralph.