I use
Blue Beacon Truck Wash a couple of times a year. Guys with brushes and pressure washes similar to the ones in a regular car wash.
As others have said, you will need to wash your roof first, as the wash guys don't climb up, or work from catwalks. There is just an overhead rinse rack that sprays off the roof, so any gunk on the roof will wash down on your nice clean rig.
For a couple of more bucks, they will spray some kind of liquid protectant on it.
They wash big trucks that have paint jobs that are worth more then some small houses, and that have big hairy drivers that would be real annoyed if the paint job got damaged, so these guys know what they are doing.