You can use a normal Car Park provided the vehicle fits into a standard size bay - remember that if you exceed the allotted space you could be fined.
You could also try Windsor Coach Park, but they may be full in the high season. They do take advance bookings.
By telephone: 01628 683880
Email enquiries to Parking - CSC
By post: CSC (Parking Section), RBWM, Town Hall, St Ives Road, Maidenhead, Berks SL6 1RF