Well-could be the dreaded black mildewy stuff that plagues almost all campers. That's easily removed with the camper/RV mildew remover wash. Comes in a spray bottle and is effective.
Or-the gelcoat was improperly applied and is discoloring due to UV exposure, moisture, something trapped underneath. This would require more serious intervention.
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW