Be sure to look at code, added code not shown in video.
Code File: https://bit.ly/3r5Qqwc
Idea: Move to the first and second room, while you're inventory stays the same as collected. Beat the boss with the items you collected. You beat the boss, and you move to the next level, where it will save automatically, then press the load button, it will load your room after the boss, but this will not include your inventory. You're inventory for the next level will be zero when you load it, also after you beat the boss, while still playing the inventory will be zero too.
The reason you want to set all inventory back to zero after the boss and loading it, is this. Place o_global and o_draw back into the room, this will reset your items back to zero, its because, if you try to load Persistent item count numbers and images, the numbers and images will not be there, because you did not put in the o_global and o_draw objects in the room. Persistent only works, when playing game, not returning back to game at a later time. You could have the images and numbers carry over like you did for the first and second room, but the problem will be when you load the game, and inventory will not be there. That's why counts are reset to zero. Plus it makes it more of a challenging game.
Please Watch Previous Video's on Easy Inventory, to setup you're Inventory System.
I did open up the loadit script and talk about it, where if you did do a save it will read the loaded room, but did not open up the object for the load button on the video.