Another item that is easy to lose is your propane tanks, especially the new type like ours that only require your hand to tighten the fitting to the tank.
I have not lost mine yet, but I don't intend to either. I installed a 1/2" eye bolt with large flat washers on each side on the back wall in the propane tank compartment. Before I installed it I spread the eye enough to get the plastic coated 3/8" cable eye into it, then I hammered it shut and spot welded it.
The cable is 4ft long. Thread it through the top handle of the back tank and through the front tank. I drilled a 3/8" hole on the side of the handle on each tank for a padlock, so that which ever tank ends up on the outside position, it will have a hole for the padlock. The end loop of the cable is attached to the lock.
Since we are not allowed by fire code to lock the access door to the LPG tanks, this is my first line of defence, if that dosen't work I will tie the Doberman up next to them.