Hi Paul
I have installed a 150 watt inverter in my caravan to provide mains power for my Sky Tv freesat system, when not hooked up to the mains.cigar socket lead direct to the fused outlet wire from the battery .
I did it this way :-
Retain the cigar socket plug on the inverter.
Buy a cigar socket lead from Towsure or similar (see Towsure website product number A26 or SE5)
Wire this directly to the main 12 volt fused supply wires.
These are found inside the caravan near to where the 12 volt supply leaves the battery box.
Connect to the fused +ve wire so that the supply to the inverter is fused.
Maintain correct polarity as this is essential for the inverter.
Place the cigar socket in a place for easy access.
On my Swift the battery box is in the front offside corner and the socket is above laid flat on the window shelf.
The inverter also sits on this shelf as it needs to be kept uncovered for cooling reasons.
Only connect the inverter to the appliances you are running not to the caravan circuit.
I use a 1/2 mt. 4 plug flat extension lead plugged into the inverter to gain extra sockets.
I plug the Sky box and the TV into this, and the total load is 100 watts.
The inverter is 90% efficient as yours will be.
This set up does not drain the 12 volt battery any more than a 12 volt spotlight
With such a setup you can use your laptop in the caravan or your car, simply take the inverter and laptop and plug it into your car cigar socket for car use.
Hope this helps.