Chopper-
You can find everything about the construction of the 2017 Premier here:
https://www.keystonerv.com/media/2261/premier-2017.pdf
That is the Keystone brochure that breaks down the construction materials. Looks like your roof has 5" galvanized roof trusses. It does not specify the decking material. If no ladder, not walkable. You can easily maneuver around with a piece of plywood big enough to stretch across trusses. That's what I do on my Premier.