HELP - Soot on our sidewall... water heater?

I have black soot coming up the sidewall of the trailer from the water heater vent. I assume this is a flame issue but is the first steps to figure out the root cause? I can hear the burner turn on so I know the water heater is working, I don’t know what is causing the soot to build-up!

Sticking to electric for now.

I assume you mean something like this?

This is fairly common, the root cause is the burner does not have enough oxygen. If you look at this example, the large opening on the right is the tube that flows through the tank to heat the water and where the burner flame would be. The smaller tube inside of that, follow the 90 degree bend and you have a sleeve. This is the air shutter, most likely it did not move but that could be one potential issue. The other issue could be bugs, especially spiders of which some are attracted to the smell of propane could have nested or be blocking the vent.

I would get a can of compressed air like this, and start by blowing around the air shutter. Then blow out the burner tube but trust me from experience, wear safety glasses or stand far to the left side of the water heater. When you start blowing air, if debris comes out it will blow all over you and in your face, can be a real mess!

https://www.amazon.com/Compressed-Best-Elements-Non-Toxic-Bitternt-Electronics/dp/B07G1SWB17/

As far as prevention, especially in storage I have the screens over my vent covers. It won’t help for everything but does make a pretty big difference. They just have a spring wire that you use the included tool to clip on to one of the louvers and holds this inplace.

https://www.amazon.com/Camco-Screen-Protects-Stainless-42145/dp/B00192JFI6/

For what it is worth, I also have them on the fridge vents. It’s cheap insurance.

https://www.amazon.com/Camco-Flying-Insect-Dometic-Refrigerator/dp/B0024E6UE0/

Yes this is almost identical to what we have. The soot on the side of the trailer cleaned up easily but was much darker. That is what scared me so much. So I assume I am talking about blowing this out when the burner is NOT on, right?

Correct when the burner is not running, I think that would be obvious.