Road to the Multiverse - S8-E1
Other mistake: In every universe - except for the double-headed universe - Brian and Stewie travel to, they (seem to) replace the version of themselves that universe already has of them, but then in the Dog universe they have a separate version of themselves that they continuously interact with.
Other mistake: A bank wouldn't lock someone in the vault all night. Employees make sure customers have left before locking up for the day.
Other mistake: Joe salutes Quagmire's Dad. His legs are paralyzed, yet he uses them to push himself backward in his wheelchair.
Chosen answer: It's a real song called "Elvira". Originally by Dallas Fraizer in '66, however, this version seems to be the Oak Ridge Boys cover version. However, the mega lesbians skip the verse and just sing the chorus after the opening line.
Bishop73