Possibly the voltage down-convertor would have some efficiency loss in the down conversion, resulting in greater overall battery drain.

About the only thing you can really do, and you have to do it VERY carefully, is spread the load across the two batteries by tapping each one for 12 volts. This is dangerous though, because you end up with two grounds, one at 12 volts higher potential than the other.

So, if you touch two things together that are being powered by different 12 volt taps, you get sparks.

My recommendation is to leave it alone, or get a bigger capacity battery for the one that takes the 12V load.

It is a system of compromises. They designed it to be safe for the most part, rather than get the most out of the batteries.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles