So, Jerry. That had me scratching my balding head.
I got out my HP Pavilion DV7 laptop and took a look.
The battery is an 11.1 volt 100 Watt Hour Extended Run Li-ion battery (Model number HSTNN-LB2W) and it runs the laptop just fine not plugged in.
The Transformer that charges it (and runs the computer) outputs 18.5 volts DC at 6.5 amps (120 Watts) to charge that battery AND run the computer at the same time. The AC load at 100-240 volts (rms) is 2 amps or 240 watts at 120 volts to produce that stable DC voltage output.
Thus a 12 volt cigar plug hooked to a source of DC (Converter or Battery) capable of delivering 120 Watts (10 amps) connected to a plug of the correct polarity should (will) run my Laptop just fine with no inverter or Transformer loss.
In fact here is a source for the correct car charger for my laptop. It boosts the 12 volts to 18.5 volts and only uses a measly 3.5 amps or 65 Watts for 21 bucks. HP Compaq Pavilion DV7-2177CL Car charger - HP Compaq Pavilion DV7-2177CL Laptop Car charger, HP Compaq Pavilion DV7-2177CL Car charger Price Online in US
Now it will take forever to charge my Li-ion 400 Watt Hour battery, but it will in no way destroy my camper's house battery at 3.5 amps.
There is nothing in ANY computer (laptop or desktop) that uses 120 volts except the main power supply. All components use either 5 volts, inverted 12 volts (-12 VDC), 12 volts DC or some combination (like hard drives and CD-ROMs that use +5 and +12 VDC - 5 volts for the control board and 12 volts for the motor drive).
Some newer power supplies ad 3.3 volts DC for the newer low power processors:
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW