I've had good success using Clorox Cleanup.
Spray on, scrub with stiff bristled brush, then spray again and scrub with soft bristled brush. Rinse thoroughly. Make sure you rinse the sides of the trailer as soon as you get off the roof.
'13 Excel Winslow 34IKE
'12 GMC Sierra 3500HD
Previous RV's: '03 MobileScout Titan 29DBS; '01 Sunnybrook Lite 2708 SLE; '99 Coleman Utah
Previous TV's: '02 Chevy Suburban 2500 8.1 4.10; '99 Chevy Express 1500 5.7