I've never heard of anyone having problems with theft, other than maybe teenagers taking beer out of a cooler. And we've left a "boom box" outside our camper in plain view (under the awning) when we've been gone all day in the parks, and it was always there when we came back. For the most part, I've found that campers, and particularly DISNEY campers, are some of the most friendly and trustworthy people around! I guess I wouldn't trust this theory with something TRULY valuable, but then again, maybe you should leave such things at home anyway. (i.e., I wouldn't bring any heirloom jewelry with me on a camping trip, but I'm fairly comfortable that my camera won't be stolen.)
Since you won't have a car there, maybe you should check with FW directly to see if they have some kind of safe or someplace for your valuables. You can call them at (407) 824-2900. But again, I wouldn't worry too much about leaving your suitcases and stuff like that in your tent. And won't you have your pin bags and camera with you? (You definitely DON'T need to worry about leaving things, even valuables, in your tent when you are there, and just away briefly, like at the comfort station.)