Hi Thano welcome to the Brown Cafe. You need to find out if you are permanent or temporary. You may or may not get laid off after peak is over, it depends on the area you work in and the staffing at your facility. If you don't have many misloads as a pre-loader and work well you may not have to go back to unload.
As far as union dues and layoffs I am not sure, find out who your shop steward is and ask him/her. They can usually help you with union questions and maybe give you a copy of the union book.
Good luck.