How does it get so dirty?

I can't figure out how the rig gets so dirty just sitting. Today I used the pressure washer I bought in Tulsa when Jay and Fay took us to the flea market. It works really well on the roof, not powerful enough to cause damage to the roof, but it will wash away the dirt.

There were a few places I used a brush to try to get some of the stains off. Parking under trees doesn't help, and the humidity here allows for mold to start growing. The passengers side and the awning was next. The awning was really bad where the moisture collects, the mold was really bad there, but a lot of scrubbing and the pressure washer cleaned it up pretty good.

Tomorrow we go to London to meet up with Paul and Connie....