Mike - here's a couple of tips from my Winnebago contact:
There is generally a strap on top that is accessed through the roof vent, I am not sure if there is one if the refer is mounted in the slideout, when I was in the shop we had not started installing them in slideouts yet. If it is in the slideout, I'd check inside the upper door to see if there is a strap toward the top. The other thing I have seen is that the 110 outlet for the refrigerator has screws going through the the bracket attached to the refrigerator and the bracket that is attached to the wall securing the brackets, similar to the bolts that the owner has removed. The last thing I can think of is that there may be sealant under the refrigerator holding it down, if the bead was large and not dry when the refer was set in place, if this is the case you would need reach under the front of the refer with a long blade knife.