I agree, first step is to determine extent of damage from 3 yrs of leaking.
As far as the type of sealant go to your local RV parts store and ask for their advice based on roof type.
I have a fiberglass rook and love the self leveling sealants, can be a few more dollars per tube but well worth it IMHO
US Navy Vet, Liberty Tree Member of Oath Keepers, NRA & VFW Life Member, Alaska EMT.
2009 Safari Cheetah 40 SKQ
2009 Winnebago Chalet 231CR