I have posted many times a true, proven fix for this problem. I have done approximately two dozen shower/tub units that were installed improperly without any base. It's is quite a long post, so do a search for it under my user name. It does involve low expanding foam used to fill garbage bags placed under the tub/shower pan's bottom. This can be done by simply removing the baseboard of a common wall, removing some sheetrock that the baseboad would cover up later. Yes, expanding foam can cause problems if used improperly. You must examine the area and use just enough foam to expand vertically and ooze out some horizontally. This can be done by a DIY person. Good Luck.
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