Your truck (in your profile) is a 2006. Doesn't it have some protection of its own? My 2006 Chevy does. Alarm if door opened. "Passlock" so engine won't start unless correct key is in it.
Lock your vehicle all the time, even when you are in it or when you are standing next to it pumping gas. Paranoid? Perhaps, but I still have my truck and all the stuff that is in it.
Park in areas that appear to be safe. Good lighting. NOT in a row of junky RV's with people living in them!
Does your truck have a manual transmission? Only a few people know how to drive those any more, so that might keep the entire rig from being stolen.
I added Lojack to my truck, but that really doesn't do anything except tell the police where to find the truck after the theft.
Nothing much keeps thieves from breaking the windows and reaching in. Keep all camper window blinds shut. Add homemade removable reflectix covers inside all cab windows. If thieves can't see what's in there, they may move on to more attractive targets.
Don't hang stuff on the outside of your rig. Bikes are stolen off camper racks all the time, all over the country.
I have lived in Southern California all my life, over 70 years now, and have never had a vehicle broken into or stolen. Yes, I've known people who did, so it does happen. Let's hope it doesn't happen to you and that you enjoy your visit here.