I've used both an invertor and now, a DC adapter that I ordered online.
The invertor, while it worked most of the time, had some problems keeping
up with charging the laptop.  I've had a couple of differnt ones, one with
a totally irritating 'low voltage' alarm...Beep beep....BEEEEEEEEEEEE......
  With a group 44 coach battery, that would sound off when my computer
battery was run down and I tried to use the computer without the motor of
the Van running.
  I found a DC adaptor for my Dell inspiron online for about $25-30 and
that seems to work much more efficiently, though it will stall if I run
the computer battery waaay down (watching a DVD, or something) and it (the
DC adapter) gets too hot.
  The DC adapter (resistor pack?) is much more compact than the AC/DC
hookup plus invertor.
  Don Hanson