Much of Arlington is quite safe, with car theft incidents being less than what you'll find across the river in D.C. That said, areas like Rosslyn and Crystal City, and even Ballston, can have more of an urban feel to them, with occasional petty crime that comes with having taller buildings that get empty after dark. And you are right to worry as a fully loaded SUV left unattended for two days could be an inviting target. Most of the popular hotels also have garages, rather than surface lots, and you could have height problems depending on the height of your SUV + roof rack combination.
With that context, I might call the Residence Inn Arlington Courthouse and the Hilton Garden Inn Courthouse and ask them what their parking set-ups are like (in terms of height and access restrictions). The Courthouse neighborhood is so-named because the Arlington County Courthouse, Police Station, and Sheriff's Office are based there. Given the heavy police presence 24 hours a day, crime is particularly low (but not zero -- I'd still take the most valuable items out of your SUV and shift them to your room).
Alternatively, you might check on the parking situation at the Westin Arlington Gateway, which is newer, a bit more upscale, and in what has always felt to me like a quiet corner of Ballston.