I use a ladder also. I put a foam "bumper" at the top of the ladder and lean it against the van where there are support pillars. That's how I waxed my RT roof when I had it.
My current van is not a Roadtrek. I have been up on the fiberglass roof when I was installing lighting in my garage
I would think RT roofs are as strong or stronger than mine. But, it is one thing being up on the roof when it is dry. It would be very slippery if it was wet and or soaped up. Very easy to slip.