My Outback has the arched roof also, putting a flat board on top of it would be impossible, unless I wanted to go bob-sledding off the side of the camper!
I'm not as heavy as you, but about 40 pounds over my "ideal"... which will NEVER happen. My roof is walkable, but I have no clue how much weight can be applied under a 4 inch x 10 inch foot print, all in one spot. So, when I'm on the roof, I'm always on all 4's. I never stand.
I'm too old to run the risk of falling, with the bowed roof, it's tricky standing and walking, and those rounded edges are deceiving, ... how close to the edge am I really to be safe? So I get on all 4's ... which is not easy for me either, since I'm scheduled for knee replacement surgery on July 31 this year! Getting on my knees is extremely painful. But, that's better than running the risk of falling.
My suggestion, if you must get up there, immediately get down and crawl around on all 4's. At least, you'll have 3 points of contact, distributing the weight (a little over 100 pounds at each point), and sometimes, it will be distributed over 4 points.
Then, when on the roof, sit on your keester when working, and do not stand or squat. Make sure you are sitting over a rib and you'll probably be fine.
There again, I think the roof will support your weight, but the real problem is that arched roof! Stay low, crawl and you'll be fine up there.
Edit:
When cleaning my roof, I do it from a ladder. When that rubber gets wet and soapy, it gets very slippery. That arched roof becomes slick as ice and even crawling around on all 4's, I found it extremely slippery, and a tendency to slide to the edges! After my first attempt at cleaning and realized this, I since have done it only from a ladder on the side. Once the roof is dry, I can get up there and using a squeegee, will spread the rubber roof conditioner. I stay on the dry area, working the wet toward me, until I get to the end. Then down the ladder, let it dry real good, then climb up again and now can crawl around on the dry area to get the remaining unfinished area.
That arched roof is great for water run off when it rains, but it's an accident waiting to happen for anyone walking on it.