Yes it is indeed a repair job, I would Sand not grind off all the old filler untill rust is not visable, then go 3-4 inches past the end. Fix much of the sheetmetal as possible, prime with epoxy or por15 then reskim the roof. Much harder then it is written. The roof has to be the hardest panel to repair on a Z. Althoe the poster above mentioned filler not being thicker then your finger nail. Why would you say that?? if it wasnt thicker then your finger nail, why use filler?(unles you have some really thick nails) Wouldent you be better off useing a high build surfacer primer? Modern fillers can be used up to 1/4 inch (not recommended) with out any problems, as long as the substrate was properly prepared. In my opinion anything up 1/16 is a good reapir and up to a 1/8 inch can be used in a pickle.
Oh forgot to add, Bondo is a Brand name, there are much better fillers on the market, evercoat rage extreme is awesome.