To post pics you need to post the pics to a picture hosting website, RVdotnet does not store photos, then you have a link.
Since 2008? That is a long time to not do service on the seams. They should be checked every year or two, and re-caulked with LAP sealant (never silicone, and almost never use caulk either, LAP sealant!!!!)
Sounds like you have rot.
How good you are at fixing will determine if it is worth it. If you plan to sell you should have this fixed, if planning to just drop it off at the junkyard later then you might can get by.
To fix, it sounds like you will need to find a barn or somewhere to get under a roof.
Yes, take the entire roof off, replace with whatever they built it with or go one better if you can.
The plumbing vent covers likely disintegrated from the UV rays.They get replaced every few years because of the sun, unless you store it under cover.
The roof will not be a fun or easy fix. It might be easier to make that junk yard trip and go shopping for a new or used camper if that is your plan soon anyway. You can spend more than the camper is worth to fix it if you pay someone, and if your time is valuable might not be worth to DIY.
Me, I would fix, but that's just one person's story.