99.9% chance it's a leak around the fantastic fan. The black powdery stuff on the "mouse fur" could be either leaks or condensation.
Resealing should be 1 to 2 time per year task if you don't live in a desert and store your RV under a roof. If you don't definitively know which seams, which vents, etc. aren't sealed properly it's a crapshoot to know whether you'll get them all or not. The absolute best way to find leaks is with a positive air pressure test. You close up everything and increase the air pressure inside of your RV while spraying soapy water over the outside - panel by panel - foot by foot. Mark all of the areas where there are soap bubbles and when dry, get out the vulkem, tempro, creeping crack cure, and reseal the common and uncommon targets. (Dealers have a special fan that hooks onto a fantastic fan vent & they ordinarily charge about $200+ for the test. Then you pay by the hour for the re-caulking. Don't let anyone but a really good AIRSTREAM mechanic do this work and make them show you that they're using the right materials.)
Do it yourself? If you have a big industrial drying fan and can make a sealed funnel from the fan to the "garage door" on your Airstream you can pump enough air into the trailer to show leaks. Use duct tape to seal the stove vent, etc.... make sure you get every bit of the adhesive off the aluminum quickly or it'll leave residue and marks.
Common things that leak:
- factory skylights
- tv antenna
- bathroom vents - plastic also deterioriates-they last about 5 years, replace with aluminum ones if you can find them
- fantastic fans
- awning bracket screws
- running lights & screws around running lights
- screw for window protectors, especially the sill over the center segment
- every window frame
- the seal of the window to the glass - maintain the seals with silicone spray
- an occasional loosely bucked rivet - found two that I could rotate with gentle pressure of one finger
- every single seam
I also use a moisture meter once a month to check the perimeter of the floor. That's where leaks turn into BIG ticket repairs!
Condensation: it is ALWAYS a problem and more of one than I used to realize. Understanding how condensation happens and ventilating to minimize it is very important. Heat water in the kettle on the stove - even with the fan running some condensation ends up in the trailer. Take a shower and even with the bathroom fan going full blast, some water ends up in the air. If you're alive you breathe and sweat, etc. If your towels dry in the bathroom the water goes somewhere. Temperature differentials can cause the walls to sweat.
With a bare aluminum interior IMHO, condensation between the inner and outer skins is worse because moist air from the inside can very easily creep into the inside seams and precipitate into sweating walls BETWEEN the walls. The Airstreams with vinyl or mouse fur on the interior walls seal those walls a bit better and offer a bit more insulation/thermal break.
Condensation at seam lines causes the black streaks on the mouse fur. It's aluminum oxide - the corrosion raw aluminum gets. Vacuuming helps remove some of it. Double check with others, but I think some people have used Oxy-kem lightly and carefully to treat these seams too. Make sure you avoid as much condensation as possible to keep the problem from coming back. Open the windows and vents whenever you can.
One thing I did not seal on the outside of mine was the very bottom of each window frame or the bottom of the midline trim wrap. If water drips outside from there - that's a good thing.
If you don't like getting up on wobbly ladders - one of the cleverest ideas I ever saw on the forums was two men polishing the roof of an Airstream by parking a box van on either side of the Airstream, and putting planks from one box truck to the other. They simply laid down on the planks and did their thing. Handy way of changing out an A/C too I'd think. In the past I've used scaffolding - but every year older I get makes it more and more tempting to let the dealer do it.
Don't work on your roof on a sunny day unless you want to be blinded AND fried! "Shade tree mechanic - is Smart Mechanic in this case.