With late model coaches, the generally accepted position is if the RV was equipped with a rear ladder from the factory, the roof is "walkable". There are many ultra light units that are built with 1/4" thick decking that are considered "non-walkable". Keystone's Bullets and Passports are such two model lines.
However, going back 15 years, I think roofing materials were generally more robust and could handle the stepping weight of an upright adult. Decking may be 3/8 inches thick, kind of the industry standard for today's walk-on-roofs. This could be the case of your Outback, even though it didn't come with an OEM roof ladder.
A work-around for "thin decked roofs" to complete required regular inspections, maintenance, and cleanings is using an overlay material (like 2" thick foam board) that will span over the rafters and support a person's weight. I purchased a full 8x4' sheet and cut it down into equal 4x2' sections and use that to crawl around on. The foam board is soft enough to protect the rubber membrane but firm enough to disperse my weight.
With your roof being over 14 years old, the bigger concern is proper inspection of the roof penetrations and seams. I don't know how long you've owned your trailer. But with an aging coach, it's imperative that routine inspections be performed. That's nearly impossible from an adjacent, a-frame/extension ladder.
By the way, welcome to the site. Happy trails.
2017 Keystone Bullet 308BHS in Saddle.
2017 RAM 2500 Laramie Mega Cab 4x4 Hemi 6.4L
2008 GMC Yukon XL Denali (SOLD)
Hensley SwiftArrow Control Hitch with 1000 lb Spring Bars
Me, DW, (3) little DS's, and 1 rambunctious Boston Terrier