My suggestion is to not assume that the brown spots are the result of mold or mildew. Instead, investigate the possibility of mineral deposits, tannin stains, etc. Perhaps try some vinegar, lemon juice, diluted ammonia and so forth. Give it some time to work and remember the bit about the small, inconspicuous area.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.