Appalachians
A twelve letter word for working well together is the word "cohesiveness".
it's three words, but: runs batted in?
"Alliteration" is a twelve letter word.
There isn't a twelve letter word, but if you add another letter "n" you get: entertain met (add letter "n") entertainment
factionalism
Twelve and eight hundred sixty-three thousandths.
considerably
overabundant
understating
chit chatting
environmental
unsuccessful