I have decided to do the same with a Dell Studio 1557 Laptop. Wanted to charge while under way, as if wild camping in Scotland, then no power available, I got a mobile charger for the Dell from an eBay seller. Had some initial problems, because the first charger, although supposedly for the Dell didn't work (design fault). Sourced another from a second eBay seller and this worked OK in the car, but not on the 1200GSA

.
Eventually realised that the charger was 90W (7.5V @ 12V) so CAN-Bus didn't want to play, as over 5A.
PopEye (of this parish) made up a special lead for me to connect direct to the battery and supply a PowerPort in my side box. So I am now all set.
If you want to charge on the go, you will need to check what your charger power demand is and may need a direct connection to the battery (if your auxiliary power socket is powered via a CAN-Bus). PopEye reckoned 12V cigarette sockets on bikes can be rather unreliable so the alternative direct cable with in-line fuse gets around this problem.
(I wanted to have the Laptop available for photography editing, etc).
Battery on the GSA should be OK as I don't always use the Fog Lights so that saves 110W and the Laptop charger will probably only need to charge for circa 4 hours max.
Grey Beard