Well, I gave the test server a quick try-out, with a fresh save. Zipped through the tutorial, grabbed a crane for the Fleetstar, put a low saddle on the GMC*, and went to test out the truck-to-truck loading on the pre-loaded semi-trailer in town. (* The naming on all the upgrades and trailers is completely screwed at the moment. Had to install a "Dangling bucket", and apparently this is a gooseneck trailer now. Also, sidebed trailers are now trains )
^ This is with all cargo unpacked - First I rearranged it all a bit, bunched three of them tightly up against each other on the semi, and threw two into the sidebed. Then, without packing anything, I jammed the Fleetstar into the side of the semi-trailer, extended the anchor (which embedded itself partly in the tire of the GMC), and then shoved the crane into the top of one of the loose cargo items.
I switched back and forth between the two trucks several times, packed and unpacked a couple times each, and not once did anything drop through anything else.
Seems promising! This is but a single test case... so I suppose it could just be a sheer fluke, but hopefully not. Can anyone else rig up some extreme test cases and attempt to confirm?