The route that water takes to get in to that area is actually very tedious. I found it by accident one rainy day.
Rain runs down the seam between the upper rear panel above the rear wheel and D-pillar, enters the seam, appears inside the cavity directly above the rear wheel, rots out the bottom of the D-pillar and also runs forward over the wheel arch and into the bottom of the C-pillar and rots through to the area above the jacking point eventually damaging this too.
Make sure all your seams are sealed up tight!
P.s, sadly I concur, not safe to drive. I wouldn't even like to bodge it as you can't really guarantee the strength of the repair section. There's not an easy way into this area either, you have to go through the floor inside the bus or wheel arch panel off and access from outside
