It can be quite difficult to hand wash the roof of the van, depending on what model you have. The one we are washing here has a lot of intricate areas, solar panels, vents, and air conditioning units that prevent any kind of effective hand washing (even with an extendable wash tool pole).
Fortunately, the roof here was not too dirty so we could get away with a snow job and rinse down.
Some car wash facilities will have a dedicated wash bay with a gantry that you can walk on and safely access most of the roof.
We do not recommend climbing on the roof of your van, some vans are not able to support the weight, and even if it can, the risk of slipping on the wet and soapy surface is too great to be worth the risk.