I invested in a lightweight scaffolding platform that has wheels and I roll it around the van so that I can wash the complete roof.
That way I do not need to protect the van from ladder dents as nothing touches it at all.
You could always get a folding platform ladder as that would do the same job.
I know that our van is small but I took a small ladder inside the van I then opened the skylight put my bucket on the roof and then put my head through the opening and from the ladder I reached outwards and was able to clean all around the skylight. The rest of the roof was done from a ladder outside. After I had closed the skylight of course