Is more than one deck of cards ever used in Phase 10?
Yes, we have had an instance where we made it through the draw pile without anyone going out. (There were 5 of us). We used to play a lot, so I think it's rare. We just took the discard pile (except the top card) and shuffled it to make a new draw pile.  I didn't see this situation in the rules though, so it's unofficial.