I'm a fan of having a car alarm that you can trigger via key fob. You can get inexpensive ones off Amazon that should meet your needs. I tripped mine once while stealth camping in my old truck when I heard someone walking around my truck in the middle of the night and they left quickly. If you can't afford an alarm system, you can buy blinking LEDs that simulate an alarm. It would be pretty easy to mount a couple of those inexpensive motion detector lights also. Try to make sure there is nothing visible in the cab that would attract someone's attention if they glanced inside- that means nothing on the seats or dash that they could smash and grab. Cover up anything behind the seats with clothes/trash/etc. I know a lot of folks in cities will leave their glove boxes open so anyone walking by can see there's nothing in them; I do this myself when I'm in a city of when leaving my truck at a trailhead. I throw an old towel over the dash to protect it from the sun and it also covers the stereo.
There are plenty of ways to secure the rear hatch on a camper shell, in fact there was a thread on expo not too long ago about it (too lazy to search). Currently I'm just using a length of paracord. Anyone trying to yank it open is going to wake me if I'm in the back; I keep a Surefire flashlight in the back that should blind them as well as some other implements that could be pressed into service. While I'm an advocate of public hangings for thieves, shooting someone breaking into your vehicle is going to be a legal nightmare no matter how in the right you are. You could easily mount some metal bars or mesh to the inside of your camper windows to further deter someone from smashing a window and reaching in.
When I'm away from the truck I tend to leave the back kinda messy i.e. sleeping bag rolled out with blanket cast to the side covering my junk and clothes laying about (I use clean clothes but to the casual observer it looks like I just tossed my dirty clothes in and not many folks want to mess with someone else's dirty clothes). Don't put anything on your truck that might be considered advertising such as camera logos, gun stickers, expensive outdoor equipment, etc.