There isn’t a statistical distribution because the loading / unloading depends on the abilities of the driver, the location, ground, temperature, and much more.
- ground: concrete floor or sand,
- location: aisle with, height of a rack cell, the guess if the item is placed well,
- driver: experience, health and age of the driver, psychologically pressure,...
- ...
You need real data of the situation. Then you can use the data to estimate a statistical distribution or use it as an empirical distribution.
