I think that the best way to send a letter is the old-fashioned way of writing a letter and mailing it. Emails can get glanced over, but a letter is tangible, and would be taken more seriously.

I would write a letter to the GM of the property you're staying at, and CC it to Marriott Guest Services (look up the address online at Marriott's website).

I am sure they would want to know what went on! Hope everything is okay.