In the Files section of the Forum (look in the second, thin, brown bar near the top of the page) are two manuals for your Atwood 8535 furnace:
A PDF for the furnace
https://www.montanaowners.com/forums...?do=file&id=26
AND
A Training Manual with lots of good information:
https://www.montanaowners.com/forums...?do=file&id=54