Hi Geoff,

You and others are quite correct regarding the ineffiencies in using an inverter to get to 230vAC, then a separate power supply to get 19v.

Points worth noting:
1) Your existing plug-in adaptor is probably not a step down transformer, but is far more likely to be a switch mode power supply. These really are quite efficient, small and lightweight.
2)If your existing power supply is 19v it may well be worth buying a 6v (motorcycle?) battery and wiring that in series with your car / caravan battery. 12v+6v=18v, but...
3) When you consider that fully charged lead-acid batteries actually supply more than 12v, you'll probably find the actual output of this 2 battery arrangement is nearer 20v.

Good luck!

John