If you happened to own a Keystone Outback, Keystone specifically states on their brochure that the roof is fully walkable.
Outback brochureIf you own a Keystone Passport, the brochure makes no mention of the roof being walkable, therefore I think it would be safe to say that a Passport roof is not walkable.
Passport brochureAll makes and models are not the same regarding roofs being walkable. Our KZ roof (alum. trusses with arched interior ceiling) is walkable and does not come with a ladder, but a ladder is optional.
Personally, I don't like the idea of a vertical ladder to get onto the roof and would never use one attached to an RV. Leaning a ladder against the side (with protection at top to prevent damage) is safer IMO. Falls off ladders are the no. 1 cause of personal injury at home or on construction sites (DW works for our workers comp. board).