It really comes down to luck. I stayed in Hostels for 3 and a half months and rarely used a locker, I didn't have any troubles.
I'm guessing some campsites have lockers but if they don't make sure you have a lock on your tent, and keep padlocks on all your zippers. If you take a cable lock it helps if you padlock the back packs to each other. This will deter thieves or slow them down at the very least.
If you have a bit of cash to spend a pacsafe
might be a good idea also.