I dug up the Atwood manual, for their water heaters.. That PDF can be found HERE
Refer to this picture, when reading the instructions:
Spiders, mud wasps, and other insects can build nests in burner tube.
This causes poor combustion, delayed ignition or ignition outside
combustion tube. Listen for a change in burner sounds or in flame
appearance from a hard blue flame to a soft lazy flame or one that is
very yellow. These are indications of an obstruction in burner tube
(FIG 9-C). Inspect and clean on a regular basis.
a. Remove air shutter screw (FIG 9-A) and slide air shutter (FIG 9-B)
down burner tube.
b. Run a flexible wire brush down burner tube (FIG 9-C) until it is visible
at end of burner tube.
c. Vacuum burner where it enters combustion tube.
d. Return air shutter to original position and replace screw.
e. The orifice, burner tube and shutter must be aligned so that the shutter is not
binding on the air tube.
HOW TO CLEAN PILOT MODELS
1. Check main burner orifice and pilot assembly for contamination
(dirt, spider webs, etc.).
2. Clean main burner tube with small brush.
3. Main burner adjustment - open air shutter 1/4 way.
4. Low pilot flame. Check for contamination - if clean have the pilot
As far as cleaning the rust in that area, you could wire brush it, clean the dust up and spray paint. Be careful not to paint anything that wasn't painted before...