If you just want to run a laptop, though, the device which I've suggested is much smaller and lighter than an inverter - I keep it in the case with my laptop. I actually have a 300w inverter in the caravan, but only tend to use it for specific applications.

I would expect that the efficiency of using an inverter to knock 12v dc up to mains and then using your laptop power unit to knock it back down again would be less than the single adaptor - when I checked it out of curiosity, the input current was only very slightly higher than the input - about what you'd expect for 13.2v (a typical battery voltage) to 15.

If you want to drive a printer as well then perhaps the best bet is an inverter - I would recommend a 300w one at least. Luckily my little cannon printer will run directly on 15v the same as my laptop.