Thank you all for posting here. This just happened to us over memorial weekend. The Mr. Clean magic eraser did not work for me. What DID work was Simple Green Automotive Cleaner Degreaser. I just sprayed it on and wiped it off quickly with dry cloth. (I also got extra cloth, just water, and wiped off any extra cleaner, that stuff is STRONG).
Now I have to go wash the whole camper to make it match the EXTRA clean spot. I'm just especially glad I no longer have smoke stains on our camper. That looked scary!
Before I use the gas water heater we will do some service to it and watch those you tube videos.
now it's all clean! woot!