There are several you-tube videos online that show how to adjust the air mixture on an Atwood water heater. I can't say that I'd trust "any of them" to be accurate, because I haven't seen most of them. All it takes is clicking on the one "bad info video" and you can mess things up. That said, here is a link to the factory pdf file on how to maintain your Atwood water heater. In it you'll find the correct way to check/adjust the air mixture.
http://www.atwoodmobile.com/images/W...ceTri-Fold.pdf
The air mixture seldom needs to be adjusted. It's a mechanical adjustment and once set, it just doesn't move (altitude is the exception). So, chances are, if your flame is "yellow enough to soot the side of your RV and leave black stains above the water heater exhaust port, you might need to adjust the air mixture. Usually, all that's needed is a nylon bristle bottle brush and some compressed air (30-40 PSI is enough). Brush the components gently, use compressed air to remove anything that might linger and you're done.