In line with what JD noted, if you first use a 12 VDC to 120 VAC INVERTER and then plug your DeWalt 120 VAC to 18 VDC Charger, YOU HAVE WASTED SOME ENERGY in the form of heat losses since none of those devices are 100% efficient. But ifffffffffffff you can buy a 12 VDC to 18 VDC charger (it still has to have some inefficient electronics for 12/18 conversion) you would be better off as far as saving the trucks battery energy...
So ifffffffff they make a plug into cigarette outlet charger to charge 18 volt batteries (still needs some electronics to do that) ??????????????????? Id go that route, however, if not or exepesive or hard to find, its true a cheap plug in Inverter in the 200 to 400 watt range (around $40 or so) would be quick n simple into which you plug your regular DeWalt 120/18 charger...........Plus the Inverter is there handy to charge youre regular 120 VAC cell phone and laptop etc if you dont have a 12 volt charger device?????
Lots of choices, do some shopping and consider youre needs, heck the 400 watt 12 VDC/120 VAC regular plug in Inverter would be simple n handy to have, its what I use in the RV so I can watch TV or charge my devices etc etc when Im dry camping with no hook ups
John T THANKS VETERANS