What holds the roof? There is a steel u shaped beam half way down the motor home. The u shaped beam is laying on one side. This shape supports the roof. I can tell you there is no flex in the roof when I walk on it.the slide it's self has a very rigid frame in it to support its weight and structure. I know it doesn't flex very much.If at all.
How it would do in an accident? depends on the accident. Structure wise you got a reinforced opening and a reinforced box sitting inside that box. I would think it would be as strong if not more than a regular wall because of the box in a box.
Saying that I did have the rollers on my center support beam fail for my full wall slide. Other than that after 7 years and the last 4 full timing. That is the only slide structure problem I've had.