Hi Kylie,
We have run into an issue while attempting to sideload a tpk file onto an Android tablet in Collector Classic version 18.0.3. When a .tpk file is placed in the ArcGIS_Collector folder created by the app on the sd card, it does not recognize the .tpk and does not move the file and generate a JSON and thumbnail as expected. I have only gotten it to do this on the internal storage. It does appear to recognize the tpk if you create the ArcGIS_Collector folder in the root folder of the SD card. It does not generate a JSON or a thumbnail for the tpks in this folder, but it does let you select them for your maps. This has lead to hours of frustration and confusion. It might be nice to specify that the folder must be in the root folder of the SD card if this a requirement for Collector to recognize it.