I stay in Walmarts all the time and never had the slightest issue. However I do try and park in a lighted area and usually with other RV'ers.
For secuiry I have bear spray if I really need something, however the best idea for trailer/fitth wheel combo's is to keep your tow vehicle keys handy and press the "painic" button on the key fob if you need to scare someone off. Also have the phone ready and like another poster's idea of writing down where you are for 911 calls.
I usually have my motorcycle in the back of the pickup and it has an alarm on it with a flashing light so that should also warn would be thieves off.
