I can enter any number in the "Mega Items In Storage" Trainer option, but it will immediately switch back to 0. When I use this option in game, every item will be at 20000 overflowing every warehouse, farm, etc.
You can enter any number you like, it resets back to 20000 when ur done using it, if you want to do 1000 remember to click away where you enter the new value.
From a village you can buy opium, undried ganja or undried coca leaves, transport it to your dryhouse and wait untill its done drying, bring the finished product to a warehouse, use the cheat to raise it to the number you like, destroy your dryhouse. Then you got no more buildings overflowed with product and a warehouse filled with finished drugs for you to sell without the hassle.
This is how im currently using it and it works like a charm, hope this works for you aswell.