I am in the same boat (RV?), as you. I need to add a DC outlet near our bed as I will be picking up my new Cpap from the VA this afternoon. I will then plug my 250 watt inverter into the DC outlet and the Cpap into that.
When I added a DC outlet on our old TT I just ran the wire to the back side of the fuse panel. There are usually a number of unused slots. If the run is 10' or less, measure both ways, then #14 duplex multi stranded wire works very well. If you do tie into a light fixture you will want to put in an line fuse (either way a 10 amp should be about right), between the light fixture and the new outlet.
:C