John,
Obviously I keep the driver and passenger doors locked. The side door has to stay fully open, and I leave the back doors open to aid ventilation as in my van, there is only about 18" head room above the TM and it gets very hot inside. I dont leave them wide open, I bring them together at an angle (If you know what I mean) and fasten them with a bungee. Not ideal I admit, however to date Ive never had kids messing about near the van - everyone steers clear of it, and I dont see how a theif could get injured, in the act unless he tripped over the pipes.
Dave.