A lot of 24 V vehicles will have converters to produce 12 V. I've seen them used to power radios. At least one of the vehicle tracking units I deal with was powered from a 24 V to 12 V converter, in spite of the fact that it will run on either voltage.
The vehicles that Ghostman dealt with were obviously arranged to run a 12 V trailer, but I think that there are 24 V vehicles that are wired for 24 V trailers.
If Juniper is wanting to charge a 12 V battery from a 24 V vehicle, he is going to need a voltage converter, something like this:-
**broken link removed**
However, that won't work with a 12 V input, so it would need to be bypassed for 12 V operation. I would suggest a 12 V relay to bypass it, and a voltage-sensing circuit to turn on the relay when there is a 12 V input, but not a 24 V input. I would also suggest a time delay of a second or so, to prevent the relay operating when the circuit is plugged into 12 V, or if the 24 V supply dips briefly.