Keystone makes one of the largest lines of travel trailers of any manufacturer in the US. Some are "heavy as sin" and some are "light as a feather". Obviously both can't use the same materials in construction and have such a significant weight difference. Lighter, thinner material goes into the lighter trailers and with that "trade-off" comes reduced capacity in a number of areas. You simply can't walk on the roof of most "light weight, X-Lite or Ultra-Light" models. There are a large number of threads on this forum (and other forums) discussing this. One very recent thread is located here: http://www.keystoneforums.com/forums...ad.php?t=32886
Years ago, a 1/2" plywood roof over 16" spaced trusses would support quite a "large owner". Today's 1/4" OSB roof over "engineered trusses that are strategically placed" does not offer the same strength, a tradeoff made to allow larger trailers that can be towed by smaller vehicles. As that process "evolved" many owners also grew from 150 pounds to 250 pounds, so the problem was "made even worse". So, even if one owner (who weighs 125 lbs) can walk on the roof of his Premier, that doesn't mean it's OK for another owner (who weighs 325 lbs) to do the same.
Generally, if there's a ladder installed by the factory, it's OK to access the roof. But that also has significant limits and doesn't mean "just anybody" can climb up there without concern for causing potential damage to the roof.
If you do a forum search for terms like, "Walkable Roof", "Roof Maintenance" or similar terms, you'll find enough information to keep you busy reading for hours, maybe days. Doing a little research will give you the information you're looking for, allow you to become an "expert in all things about the roof" and probably answer your questions about what to look for, how to repair and maintain your roof, what to use to clean it and how to reseal it. All of those are "the evolving questions in owning an RV".....
Welcome to RV ownership, and no, they aren't "like my house, so why can't I...?"
ADDED: Some roof systems (not structures) in some trailer brands don't even use trusses. They are constructed of an inner 1/8" luan sheet, 3" of foam and topped with another 1/8" luan sheet. This "sandwich construction" is even lighter, but obviously even weaker than what is on your trailer. As trailers get larger, lighter and as owners demand more "conveniences" be installed (adding more weight), the actual structural integrity will probably grow even weaker than you see today.
I remember sitting on the hood and the roof of my 1950 Chevrolet. I'd never dream of sitting on the hood or on the roof of any truck built today, much less on the roof of a modern car. The trailer industry is evolving to that as well. Some models are already there.