You can get power supplies for 12v DC Automobile that will supply
12-18-24v for your laptop.

I don't think you could both use the battert current and at the same time
get any charge back into the battery, it just sounds like a loop thats
got to have a flaw in it somewhere.

You might consider an additional battery and/or running your engine for
about fifteen minutes three times a day to get the charge built back.

I've used inverters to run 110v electric drills and such from work trucks
and they work very good but you must by the amperage required for your
peak needs.
Your computer will require at least 300watts.
TV sets probably 500watts.
But electric heaters and battery chargers would require 1500watts and be
expensive.

Stan Wilder
83 Westfalia Aircooled