I believe the reason your manual says that no maintenence is required for the roof membrane is that many folks tend to use harsh chemicals and treatments that tend to over time remove the chalked layers of the membrane causing it to thin.
A grime covered roof is one of the leading source of black streaks on the sidewalls and prolonged dirt buildup on the rooftop caulking shortens the lifespan of the caulk. I for one like to clean my roof regularly for these reasons plus it allows me to regularly inspect the caulk seams for cracking, leak sources, etc. I also use dawn dish soap and a soft brush. This is particularly important as I keep my rig parked under trees.
I believe roof conditioners are expensive and unnecessary.
Steve Rinker
Rochester, NY
2013 Keystone Sydney 340FBH 5th Wheel, 12,280 lbs loaded (scale)
2015.5 GMC Sierra Denali 3500, SRW, Duramax, CC, Payload 3,700 (sticker- not scaled yet)
Take my posts for what they are, opinions based on my own experiences.