I realize this is old, but....
I did this in an '03 Bounder. Panels on roof, batteries in the original location under the steps. Actually, my controller is in the luggage compartment next to the steps, so the cable goes into the cargo bay and then into the battery well.
The basic route was to drill from the fridge closet into the area under the kitchen, and then down into the cargo compartment. Except I worked in the other direction.
The kitchen cabinets have a false floor under them, so I drilled up from the cargo bay, then fished the wire up that way. It was much easier from the bottom up, I used a fish tape to push it past the false floor until I could reach in and grab it. Then it was easy to fish it up through the fridge area.
Remove all the drawers and vent grills from the kitchen, makes access much easier.
My Bounder is a 32W, so a longer coach might have a different kitchen layout. But the Bounder and Southwind should be nearly identical.
Hope this helps.
2003 Bounder 32W + 2011 Subaru Forester