We hooked to the Batwing. If you get the Pro Mobile kit, it has the hardware to install onto the batwing.
Cabling is a pain, but here's where I did mine. I have a 03 International. Next to the shoilet is a false wall where the vents go up. I snaked the cable down next to the vent pipe, and re-RTVd it. I then popped it out down my the desk. The WiFiRanger forum has some pictures here.
There's also an OEM kit available that puts things all in the wall. You have to order that special, as it will only ship on new Airstreams in 2012.
Full disclosure! I've worked for WiFiRanger for some time, but have an airstream and needed the WiFiRanger for the miles we used it, so please filter appropriately.