The 1000W supply merely means the power supply can PROVIDE up to 1000W at various loads of 12V and 5V to the necessary circuit boards and 'peripheral' devices (harddrive, floppy, DVD, graphics card) but does not mean that that is actually what it DRAWS from the wall. For example, one 1000W supply advertised on the web provides
and if the DRAW by the hardware only totals 200W, a 1000W supply has lots of surplus capacity but is not DRAWING 1000W out of the wall (or camper/van). it will not DRAW the 13A max from 120V outlet unless it is providing all 1000W that is demanded by things plugged in.
Now admittedly the laptop MIGHT be using lower draw components...like the CPU chip itself might be a low-power version chip than used in a desktop, so that battery life is extended. But then again, how many laptops have video support for a 4K monitor?! And then if you are a gamer and need the video card acceleration that is in your desktop, you might have a hard time getting the same capabilities in a laptop because it cannot separately cool the video card like a desktop can (which can use more electricity consuming cooling fans mounted on the video card).