Was cleaning the roof today and was wondering what is that stuf that comes out of the A/C unit that sticks to the rubber roof and won't come off. I have had two other 5th wheels and they didnt seem to have as much of that stuf forming on the roof. Is this normal ?? Its the Dometic 15K A/C unit. Just wondering if everyone has this greenish like crud that bonds itself into the white rubber roof. Thanks