Trailer container is saving all slots in NBT instead of saving only slots with items (like a normal entity/tileentity with storage)

That's probably because I fill the slots with empty itemstacks, and save those when I save the inventory. MC likely strips out the air in the save-load operations. Will investigate to see how it handles such things in 1.12.2. The save-load code got ported nearly direct from 1.10.2 when stacks were null rather than empty, so there's likely some inconsistency somewhere that's causing this behavior.

Fixed in V15.6.0.