Motohome roofs how strong are they

Bought a swift suntor (sunseeker) last september it has fibreboard sides and an aluminium roof. It has a roof rack fitted at the rear of the van,as it has a bike rack and a ladder the ladder is on the right hand rear corner there for I would have stand on the roof to load the roof rack. I have read somewhere the roof should support a persons weight up to the overcab section,can anyone clarify this as I am dubious about going on the roof.

Jim