Yea, I have a lot of fun with that photo! Still, it's a pretty awesome photo!
To help the OP a little instead of just having fun!...
Many of the newer TT are coming out with the arched - rounded roof. I specifically asked my dealer about walking on the roof of my new Outback and he said the roof was made to walk on. Shortly after we purchased it, I was on the roof (although crawling around on all 4's and not standing! I will say that arched roof is pretty sturdy, opposed to the flat ones on my previous campers.
I thought about the plywood idea when I was up there, but with the arched roof, I don't know how you'd get plywood to lay flat. It would be like a rocking chair and probably slide off! Yikes!
I was on top of my first TT (Dutchmen) quite a lot. It was 18 feet long and quite sturdy and never noticed any difficulties standing. Then we we purchased our Springdale (31 feet long), the first time I went on the roof (there again all 4's), to clean it, near the back, I heard the wood "crack" under my weight of my foot. I quickly felt for another rafter and got my foot on something more solid, rather than between the rafters. After that, I never attempted climbing on the roof again. I always worked on the roof from a ladder. I had a cover for both trailers, and that was the biggest reason for getting on the roof (to put it on or take it off). It was a real challenge on the Springdale, to do that from a ladder.
So basically, the answer to your questions rests with your specific trailer. They are not all "created equal".
FYI, the following photo shows how much of an arch the roof has in my Outback. The camper went through it's first winter at home, and the design I think, really helped snow from piling up so high. AND water never stands on the roof anywhere now either. I think the supports are strong enough to hold an incredible weight, but not sure about the material "between" the supports (rafters). Next time I'm up there, I'll proceed with caution. Still that arch is pretty significant: