Now, I am positive that there is a chapter in the Return of the King in which Gandalf explains to the hobbits, and I think Gimli, what happened at Erebor while the battle at Pelennor Fields was happening. I am also sure that there was something in the Unfinished Tales that mentioned it. But now that I've gone to find them, I cannot.
Anyway, welcome to the 'Downs,
Gandalf! I hope your stay is long and comfortable.