We have 1809s (like yours but with sideways bed). I would find it hard to believe it would open if both locks were engaged.
That being said... I have found that you have to be careful with turning the key to lock. The deadbolt turns one way and the regular lock turns the other way. If only the regular (top) lock is engaged, the door may not be completely shut and it could pop open. I have noticed this when I thought it was locked sitting in our yard, but when I pulled the handle,the door came open. If this was the case on the hwy, after a few bumps I could see how it would open by itself.
Again if both locks are FULLY engaged, esp the deadbolt... I don't think you will have a problem. Of course, a little bungee insurance never hurts.
P.S. Trick regarding bungee cords: I use one to lift low hanging tow chains and wiring cord up a little to keep them a little more off the ground. Since it stretches, can never be a problem. Trick is to use one the right size and let it hang down loose...
2012 Ford F150 4x4, 5.0L, 3.55
2011 Rockwood MiniLite 1809s
E2 WDH 600/6000lbs