While most 13' trailers aren't strong enough to support a person's weight due to open floor-plans, areas on larger trailers that have internal support such as cabinets or dividing walls can hold a person on the roof.
I regularly clean and wax the roof of my Fiber Stream
for the reason SherryNPaul stated: It is stored outside without a cover, and the Southern California sun
plus the particulate air pollution in the city
combine to cause damage.
My Fiber Stream
has two cross-hull dividing walls (between the Bathroom and Kitchen, and between the Kitchen and Living/Bed Room) that internally support the roof while the four molded-in lateral support ridges externally strengthen the roof.
I never stand on my roof. I use a spray cleaner/wax and hand buff with dedicated towels that are only used on the trailer.