If your motor home has a factory installed ladder up to the roof, the manufacturer knows/expects you to be crawling/walking around up there once and a while. Think of the liability aspect. That said, you are a pretty big guy and the idea of spreading your weight around using plywood is valid. IMO, you'll more likely fall off the roof than through it so be aware of that. Leave the stiletto heels in the closet and wear clean flat soft sole shoes to avoid localized damage. Be aware of and avoid soft spots. The roof will more likely deflect under load long before it breaks. I'm over 200 lbs. myself and I never like the wobbly feeling of walking around up there because the MH moves around just because of my weight, not to mention the wind. Other people moving around inside or entering/exiting move my Class A around a lot, especially when I don't expect it.
Chum lee