Can't help much with the aluminum and getting the black streaks off. Some stuff will eat the finish off so I don't want to make a suggestions. I'd still steer clear of the pressure washer. On the suspension, usually 40-50 ft pounds I believe. On the roof, dicor 501 is fine whether it's an Alpha or a Dicor roof, it's usually all compatible. Don't remember which roof is on that year model. You can use Dawn on the roof with no problem, spic and span etc. Some have their own recipes as I do.
John and Rebecca Dickson
Emma-8 / Little John-5 / Iva-1+
2013 Ford F-350 Lariat FX4, CC LB PSD, DRW
2015 Jayco Jay Flight 28BHBE (#8)