How to create a Hash Computed Column for Many Columns?
Just concat your columns together rather than trying to convert them into xml and then a hash.  

CREATE TABLE [dbo].[CustomerTransactiont]
(
    CustomerTransactionId int primary key identity(1,1),
    CustomerName varchar(255),
    Price decimal(10,2),
    Quantity int,

    RowHash as hashbytes('SHA2_512', CONCAT(    UPPER(COALESCE(CustomerName, '')), '|'
                                                , COALESCE(CONVERT(VARCHAR(50), Price), ''), '|'
                                                , COALESCE(CONVERT(VARCHAR(20), Quantity), ''), '|'
                                                )) persisted
) 


Note: The coalesce and the pipe characters were just for examples. The Upper is optional if you either want hash to be case sensitive or not.