Started using Xmonad and love it, but:

I'm using a laptop that I sometimes dock. I then have the laptop screen turned off and use instead a much larger separate screen. The resolution gets messed up when I login Xmonad (I guess it retained the laptop's original resolution). I then runxrandr --output HDMI3 --mode 1920x1200 to change it to the larger screen's full resolution, and everything looks nice. But the xmobar doesn't stretch across the full width of the screen and the background desktop image is tiled to fill the whole screen... 

I guess what I'm asking is, how can I best set up .xsession and / or xmonad.hs (and .xmobarrc) to make this work?

Ultimately I'd like it to work when I dock my laptop but also when the laptop isn't docked...

Many thanks in advance for any kind of help!