You are gonna have to remove the ceiling. Ppull out all the wet insulation. I am in the process of that very thing in my 323 keystone sprinter. You must get it dried out. With all the insulation gone you can run a hose over the roof and see where the water is coming in. Place a dehumidifier in there and close it up and let it run for 2 days.
2011 Keystone Sprinter 323BHS. Retired Master Electrician. All Motor Homes are RV's. All RV's are not Motor Homes.