Just a good soap and water on the roof. Do not use Dawn on the roof if you have an exterior protectant on the fiberglass.
Otherwise I use a long adjustable pole with a brush on the end from Mr. Longarm.
Mr.LongArm Hydrasoar Flow Through Extension Pole
Mr.LongArm Inc. Manufacturer of Quality Extension Poles
and a window cleaning bucket Unger 6 gal. Heavy Duty Rectangular Window Cleaning Bucket-DB02 at The Home Depot
and this soap. http://www.meguiars.com/en/automotiv...mate-wash-wax/
Also just a brush on the roof with a mild soap and just keep it clean and it will last a full lifetime of the roof 10-15 years +.
Waxing will prove to be much harder so use a good marine wipe on wax and do small areas at a time and make sure you do not have the surfaces hot or it will be very hard to get the wax off when buffing.
If power buffing be very carfull to use the right pad and DO NOT burn through the to the fiberglass as that will be permanent!
A really good waxing 1-2 times per year will keep the exterior in pristine condition.
Somewhere Turbs had a good write up on how to clean the roof and why you should.
Oh BTW start at the top and work down!!