I parked a car there for seven days and had no problems.
Since it is well known the cars parked there are left overnight for many days in a row I would be very surprised if they were left unguarded at night.
That being said - all car parking lots limit their liability - so I would very careful about not leaving anything valuable in the car - and if it is a commonly stolen model - just make sure your insurance is up to date.
I have parked there for several cruises but never for 30 days. There is security and the parking lots are monitored and patrolled. As mentioned, lock your car and don't leave any valuables in plain sight.
Quantum of the Seas 12/2014
Oasis OTS 6/2015
Serenade OTS 12/2015