Not an ATL introduction per se, but rather a maintained introduction : Merovingian Gaul, for exemple, used to have some camels for carrying goods and treasures, and maybe as transport in warfare (while the only mentions I remember are during retreats, it implies their presence in the campaign in first hand to be able to retreat from somewhere).
I don't see them lasting more than exceptionally in Gaul, at least in first times, but what about a more widespread use in Spain, even before Arabic conquest?
Their presence coming from roman times, it shouldn't be that hard to have them lasting even a bit more in some places as other roman features did : it should be ironically better adapted in semi-desertic regions of Spain where romans roads are less present (as not that usable by camels).
The concurrence of wheeled transport would of course represent an eventual threat to the permanance of this use of camels, being more adapted to roads and non-desertic places, but it could be enough to have a lasting camelid population at least for a longer historical period.
(The maintained contact with Africa in Al-Andalus would be as well a factor of maintain, while Merovingians couldn't renew their population even if they really tried to)