Indeed. Once your ghouls are killed, they are lost. Whether they were in decline or not.
active ghouls would re enter the board just like any other race. In Decline ghouls get to reneter as well
from the FAQ:
If in-decline Ghoul tokens have been wiped off the
board, they may re-enter it on the players next
turn by conquering one of its border regions.
When in-decline Ghoul tokens are attacked; Do
they die similarly to active races (just lose 1 token
per region) or do they get wiped out?
Ghouls act like an active race when in decline,
so would only lose 1 token per region los