Pictures would help but check out YouTube for plenty of repair videos on roofs on RV's.
Home Depot has a elastomeric coating that can be put on. Here is the link. It's brand name Is Henry's. Some say that it keeps the rig cooler as it repels most UV rays and can also sound proof the rig a bit. I talked with a guy who uses this out in Arizona for shed roofs, trailer roofs, and RV roofs. He has had good results. As with any project, the prep has to be right or it will not come out good.
The cab over support will be an issue as that will need fixing. Watch the videos and see if it is something you can handle but be aware that sometimes when you open something up it becomes a bigger issue. If the supports are rotted away probably the wall is too.
Only you will know if this is something you can handle. Roof leaks can really ruin an RV. There are a lot of Videos on YouTube that will help you to decide if this is something you want to tackle.
Good luck, Lynne