I see there has been no answer to this post. I don't know if you decided to modify the structure, or not.
Have you considered picking up one of those collapsible ladders. I purchased one and it stores easily in the basement of the MH. It will access the roof with no problem. Besides being able to do that, it can be used to wash the windshield, and any side windows that you would like to do. I probably would cost less than a structure change, and it is more versatile.
Wayne MSGT USMC (Ret) & Earlene (CinCHouse)
2008 Winnebago Destination 39W
It is what it is, and then it is what you make of it.