I have run off stains as well but mine are black, like dirty water stains that run off.. at least for now they are black.
Need to get up there with a scrub brush and some spic n span to see if I can get it off. regular soap/water and a soft mop didn't do it.
Not sure which warranty you are referring to but if you are talking about the camper warranty, Dometic warrantees their AC units for 3 years. You can get it fixed/replaced if they consider surface rust a warranty item at any dealer and Dometic will pay them direct for the repair.
2015 F350 6.7 Platinum
2015 Coachmen 360IBL