Hi iamere
Try searching for DC-DC converter on Google. You need a 24V input (capable of taking up to 32V maximum) and 13.8V output to charge the leisure batteries. It also must have electronic current limiting for use as a battery charger (some converters are only suitable for use as power supplies, i.e. powering a load such as a CB radio).
A typical suitable one is here:
http://www.thepowerstore.co.uk/product.asp?ID=91
I should declare my interest in this converter in that I designed it (I don't have any financial interest though).
You have got 400Ah of leisure batteries, so if you charge them at 15A whilst travelling for, say, 6 hours then it will still only put 90Ah maximum into the batteries. It won't be cheap to get a useful size of converter.
Don't tap off the mid-point of the vehicle's 24V battery as this would unbalance it - the top 12V would over-charge and the bottom 12V would under-charge. Could be very dangerous.
Hope this helps
Kees