The truck places don't wash the roof - and you wouldn't want them to. I use them to get them occasionally to get the road dirt off the rv. I make sure they don't use high pressure - one reason I'll select Blue Beacon if I see one.
Wash the roof carefully - you do not want to cause damage to it.
There are specific cleaners that supposedly will get the black streaks off - most rv places carry them. Some swear by them, others say just use basic cleaners and elbow grease. My current rv has specific things to not use to clean the rv - so I pretty much use Dawn dish soap
The black streaks are from the rubber roof. Most important on the roof is to regularly check all the seals and make sure they aren't cracking, etc. There are products you can put on the rubber roof - usually not necessary more than annually, to help protect it. As to getting on the roof - I do every month to check the cracking, etc. It's part of my monthly maintenance "program" and also why I'll always have an rv with a ladder built in
That said, don't mistake me for a person who does her own maintenance - some, yes, but most, no. I do my own preventive maintenance the best I can!