Battery wasting via voltage regulator
The 7800 series of regulators take a quiescent current of a few milliamps.  This is probably what's burning down your 9V batteries when you're not operating the unit.  Modern LDOs can be had with quiescent currents in the microamp range.

When operating, your efficiency is under 70%--not bad for really low duty cycle, but if your on time is significant you might consider a switching supply.  Second choice would be to put the batteries in series and use a single LDO with low Iq to regulate from 18 to 13V.