RE: Powering a laptop
One thing you can do to simplify this.  When choosing a laptop for purchase,
check to see if there is a 12v 'car adapter' available for it.
Surprisingly, a lot of the laptops require either an inverter or the adapter
is in fact a small inverter.  In my case I chose an HP Pavillion series, and
the 12v adapter is just that, plugs straight in to the 12v socket, runs the
laptop and charges the regular battery at the same time.  A small inverter
can sometimes be cheaper than the factory adapter, but be careful:  Some
laptop/inverter combinations won't work together, the computer power supply
doesn't like the shape of the output from the inverter.
As far as charging the aux battery, wire it into the charging system as an
aux battery, you won't have to worry about remembering to attach the charger
when you get home.
I use a standard lead-acid battery for aux (flames coming, I know, I'll get
shorter battery life but I'm not using every day, cost to replace a little
earlier than deep cycle made up for in the long term cost), mounted under
the driver seat.  An example of use:  In a one week stay in the high desert
of eastern Oregon in August, I ran the laptop off the aux battery an average
of two to three hours each day (processing astrophotography done the night
before) each day for a week, and also ran a LCD TV for about an hour each
morning and evening during that time. (laptop run from a separate 12v
battery at the telescope for two to four hours each night at the telescope)
I checked the aux battery each day, the lowest it dropped, toward the end of
the week, was just under 12v.  I didn't start the engine the entire week.
Summary:  You will probably get a lot more time out of your aux battery than
you expect, and hooking it into the charging system solves the charging
problem.

Loren A. Busch 
I know that you believe you understand what you think I said, but I am not
sure that you realize that what you heard is not what I meant.