My take is that, judging from the detritus along the bottom edge of the fuel compartment, rain is entering either behind the compartment door or coming into it from an as yet unperceived compromisation point in the roof or wall above. It’s like a house roof leak... they can be hard to find because where the water enters and where it shows up can be yards apart. The old electrical bay “flood” on our coach was frustrating until I noted a split along the length of the rubber windshield brow; filling the crack with Lexel stopped the drip into the bay, despite BCS techs’ opinion that the brow couldn’t possibly be the problem.
However it’s getting in there, my guess is the rainwater is mixing with that detritus, perhaps a blend of rust and long-ago spilled fuel, and carrying the color down as it finds its way out of the fuel port. I’d be looking to thoroughly clean, with a toothbrush or other tools and Simple Green, that fuel neck escutcheon. Then try to access behind and above it somehow to find any source of rust from perhaps a steel filler port support/brace. Also review any seams within feet of the area above the fuel port and on the roof proximal.
It doesn’t take much. An infinitesimally tiny compromise (hiding underneath the wires) of the sealant globbed around our solar wiring roof entry point resulted in a washout of our living/galley area a couple years back. A gulleywasher came through while we were away from the coach for a few hours, and we spent several late night hours mopping up, and me on the roof in the dark rain trying to zero in on the culprit. No fun, but I found it... and the lake formed from a shallow dip in our roof that’s supposed to be “domed”. Naturally the solar wire entry hole was under the lake!
So take a real careful look at all caulked seams above, and see if you can get enough behind the fuel filler box to follow any leak trail. Depending on the exact situation, I might consider running beads of Lexel anywhere I could get it in that fuel filler box, on cleaned seams, to force any water intrusion away so it can’t enter from above or behind or exit, stained or not, down the coach side.
The obvious crud along that escutcheon bottom edge could be as previously noted a mix of fuel and rust... rust off the box itself from water intrusion through the port’s door. Check the upper edge of the door that when closed, water isn’t getting in... simply use a hose to test. Then repair any questionable area as seems practical, clean the box, use Lexel, and paint. Of course beyond the staining issue, water anywhere around the fuel fill is a threat to fuel integrity as well.
Joel
