Hello Kevin,
Welcome to the Forum.
If you remove the rear seats there may be enough room for a scooter, depends how big it is.
Also, on many models the rear seats fold up sideways.
If you remove the middle seats as well, which is not too difficult to do, there will be plenty of room.
Although securing the scooter may be an issue.
IF seeking a vehicle with mobility seat you MUST ensure where you will get the seat serviced and repaired.
Do NOT assume you can take it anywhere. Most Toyota dealers will not service Alphards.
It is essential to have a reliable, capable mechanic before you buy.