There are various external door locks, the original being that big ugly rusty bracket that takes a hockey puck padlocks, but now you can get smart/slimline locks.
Install one of these as high as comfortable on the side door, this will help prevent peeling the door. I’ve seen a van at work where the door of one van was peeled back by attaching a big ratchet to a neighbouring van and just cranked away til it opened up.
I’m not sure what to go for, I don’t want to draw attention to my van by covering it in extra locks (as there isn’t actually much value in there, its more avoiding general effects being stolen from a camper and the damage to the van). There is no point just securing the rear doors as any thief with an ounce of sense will realise there is no bulkhead fitted.
I could go for external locks, or the more expensive stainless armaplates that go around the original locks to prevent them being prised open. Security film on the front windows is on the list to put off opportunist smash and grabs, although I know it will only buy a few seconds for a sustained attempt.