I have tent camped at WDW many times and have NEVER had a problem with my general camping stuff being bothered. Outside my tent, I had chairs, tables, a fridge, a cooler, lanterns, shoes, etc. Inside my tent not locked up at all was clothing, linens, a fan, etc.
I usually bring a laptop with me and it stayed in my tent also, but it was locked up inside a footlocker. (The footlocker was also locked to a tree by using a long cable that went out a vent opening in the back of my tent.)
Other valuables were kept on my person. (Camera, wallet, etc.)
Also, if you have your car with you, just lock valuables in the vechicle.
As far as personal security, folks cannot get into the campground with a vehicle unless they are a guest there, and security does patrol the campground, and the comfort stations lock from midnite to 6am and you must have your KTTW card to access them during those hours.