Multiple things. Think of layers of defense. Keep people out, keep truck there if people get in.
1. I use a piece of angle iron welded to door and cab that cross the gap that can be padlocked. Kinda like the puck locks you see on work vans that go on the doors, but not. Hard to see in the below pic, but you can see it (kinda).
View attachment 914432
2. Chain on steering wheel. Secured with disc lock. Use high grade chain that's resistant to cutting or shearing with bolt cutters.
3. Shutoff (battery) switch. I think a battery switch should be on there anyway, as it prevents any parasitic loads from draining the battery - These trucks often sit for months so what easier way to make sure nothing is coming off the battery than to make sure it's shut off?
I put mine quasi-hidden on the battery box next to the driver seat. You won't really know it's there unless you reach for it.
View attachment 914433
Gotta think like your average thief/tweaker.
They're looking for something fun they can steal, something they can use RIGHT NOW.
The exterior heavy duty hasps and padlock are first way of saying 'This ain't worth your time'...
So, they either bust a window and crawl in, or spend 15 mins with a grinder dismantling the sheathed lock on the door.
So, now they're in the cab, but they have a chain to deal with - So, they can either grind the chain down, or cut the steering wheel..... agian, they're gonna need tools, not exactly a quick thing.
THEN they have to FIND the switch, make sure it's in the right position, and THEN find out the correct switches to get things working.
So, odds are, even if they did get in, they'll start flipping switches, and everything will be dead... they'll realize they're gonna get caught, they're already juiced up with adrenaline, and they're going to GTFO after realizing it's not gonna work.
If you're super paranoid, you can move the starter switch to a hidden switch somewhere else, or have a shutoff switch for the starter switch....
Layers.