If it does not have a ladder, then most likely it was constructed with materials that were not meant to hold a significant amount of weight. This does not mean you can't walk on it assuming you take precautions and factor in your weight.
Our previous RV was a Keystone Bullet which did not have a ladder and just had the thin aluminum truss pieces. Definitely not meant to hold much weight. When I got up on the roof, I made sure to spread my weight across two trusses as to not put too much weight on any one support member. I am only about 160 pounds with some long lanky legs so it worked pretty well for me. If you are concerned about your weight, I have heard of people constructing supports out of wood that would help spread their weight as they walked.
