Dollar Tree has over 15,000 stores in the U.S. and 99 Cents Only has about 350 stores. Dollar General has almost 15,000. I frequently shop at a nearby 99 Cents Only in Northern California and it is stocked regularly with fresh produce, and usually with organic canned goods, organic dairy or organic plant-based refrigerated items. I did not see fresh produce at my nearest Dollar Tree.
The heavy discounts seem to be driven by manufacturer oversupply or by merchandise that has a rapidly approaching Best By date.