I've got an old, Suburban NT22 furnace on the bench right now. Although your furnace is bigger & newer, I would think the same general sequence of steps would apply.
On this unit, the mounting box/hot air plenum stays in the Airstream. The chassis slides out of it once four screws are removed from the mud dauber ports
on the outside, and two or more anchor screws are removed from around the front plate. You will also need to disconnect the gas, electric, and thermostat wires.
Once on the bench, the burner log's location can easily be determined by tracing the gas line - in the lower left corner of the picture below you can see the log's gas line connection.
Disconnect the gas line at the controller, and unscrew the two wingnuts on the burner log assembly. With gentle persuasion, pull the burner log out the furnace.
The cast iron burner log pictured below had been sitting for quite sometime in a wet environment and was 80% clogged. I found a hacksaw blade to be useful in cleaning the slots before following up with a wire brush & compressed air
While you have the log out, make sure the air adjustment screw turns freely. This is a perfect time to work the issue.
A word of caution before undertaking this task: There is a special gasket which seals the burner log assembly to the heat exchanger. Around a 1/4 of an inch thick, it feels
like dense foam rubber. However, due to it's environment, it obviously has to be heat resistant. Since it will probably tear during the disassembly, see if you can locate a source for a new one before starting.