Chinook RV

Recently on rainy trip I used the water heater for a short period. Noticed on the outside of heater brown streaks around and above the heater. Was this burning or heat,dirt,rain mix.

Does the streaks come off with washing? I would check the "chimney" portion of the water heater to make sure it is in good shape, and not leaking hot exhaust too close to the fiber glass panel and cause heat damage.