There's no way to keep your stuff safe from a determined thief, especially at rest areas along the expressways. There may be people watching you walk away. But you can make your rig a less desirable target. I don't use the rest rooms at the rest stop. I have my own on board. If I need to stretch my legs, I take a walk around my rig and toad to inspect all the connections and tires, with the keys in my pocket and ALL the doors locked.
I've been approached a couple of times in rest areas by some not-so-cute "lot lizards" (I passed) and once by a couple of guys that walked up and asked for money, explaining that they already had enough money for some bread and lunch meat and just needed enough for a couple of beers to wash it down. That seemed reasonable and probably true, so I gave them a couple bucks. :)