Our previous rig had this issue (gray). It wasn't painted, it was the fiberglass gel coat that faded. Painting was the only real fix for the issue.
2001 Alpenlite 35RK
2007.5 Chev LMM Duramax/Allison
BD Variable Vane Turbo Brake
TST 507
2010 FLHTC Electra Glide Classic Red Hot Sunglo ๐