Can't help on the decal question, but I believe thats pretty normal for RVs. Might need some Aerospace 303 to protect the decals.
As far as roof, its SuperFlex from Alpha Systems. I use Dicor sealant and it works fine. Just wipe down the old stuff with alcohol and caulk over it. Could use Eternabond tape if you choose too. I usually clean the old stuff off to make it fairly level, then tape it up. Then come back and Dicor the edges for sanity sake.
Product Care & Warranty - Alpha Systems, LLC
https://www.amazon.com/303-30306-Aer.../dp/B000XBCURW