A few comments from what I have seen in restoring other campers using the Dometic fridge's.
As was said, rust in that area is common, heat breaks down the galvanized and rust starts forming when the system cools down, condensation sets in, and rust starts. That said, yours may have taken more water from above.
If you have the roof cap off, assuming yours has a roof cap and is not in a slide, there is a baffle in the boiler stack (the long tube/hole just above the gas burner). The baffle hangs down from above the top of the boiler tube, has a long rod then a spiral baffle attached to the end of the rod.
You unclick the top of the rod from the burner tube and pull the baffle all the way up and out. Cover up or remove the burner itself so rust from above will not fall directly onto or in the burner when you pull the baffle. Make sure the baffle has not rusted out/off and will come out with the rod. Do not leave it stuck in there if it is broke off. And do not use it with out it. And get all the loose rust out of the boiler tube when the baffle is out. If you have to replace the baffle or rod, make sure the new one is identical in length on both the rod and the baffle so it hangs in the right place.
Now to the burner not firing, not sure how familiar you are with the gas fridge, but it takes a good while to purge air out of the lines to get gas up to the fridge gas valve. It might take several cycles of trial for ignition to get gas to the burner to light. The controls will shut down on safety after 3 attempts and no firing feed back, then the fault light comes on. To reset, shut off the fridge, and turn it back on gas and it will give 3 more tries.
There is a small slot screw looking on/off gas valve before the gas valve that could be on your fridge. Make sure the valve is open. The slot horizontal is on.
You should see the blue spark jump from the electrode to the burner tube. One spark for each click of the igniter. And it should click constantly about every 1 second for a time period. Intermittent sparking points to issues.
You should hear a faint click/clunk of the gas valve turning on, then the igniter starts clicking.
To help more, tell us the year, model number of the fridge. Pics help too as we or you have no idea if someone modified something.
Also, your fridge does not look like it has the recall done to it. Pending your model and serial number, Dometic still has open recall on all the fridges that fell into that group. It is a free recall that can be scheduled and done by a factory authorized repair shop. Look into this as it was a fire safety recall. More on this if needed. I have restored several year 2,000 to 2006 made campers, and they all fell into the recall on certain models. Most owners who bought a used one never knew of the recall nor the original owner.
Hope this helps
John
