I camp in my DCU a lot over the year--mostly Spring, Summer, Fall. Many Times I have had to spend the night midway to my vacation spot and camped stealithly in a Wal-Mart of Lowe's parking lot. I then crawl into the back with foam mattress and sleeping bag and snooze away.
My latch assembly is identical to yours. What I did was loosen the set screw in the interior for both rods and re-position the locking bars toward the inside so that the handle appeared to be locked from the outside. In other words, the handle would appear to be in the proper locked position to anyone who went by, but in reality the rods were positioned so that the camper lid could be opened easily.
Then, when inside, I used two round, steel tent stakes like you get at any Walmart and wedged them between the bars and the tailgate. They fit snugly and were very strong. I had my wife pull on the door from the outside while I had this arrangement installed and she said there was no play whatsoever and she believed the lock mechanism to be fully locked.
I thought about many other methods, but conjured up this one and it seems to work perfectly for me, so, what the hell.
Hope it works for you also.