The main database file for RVCLB is usually called RVCareLogbookDB.rvclb. This stores all your main data. In older versions of RVCLB, the app allowed you to rename this database but this is no longer possible in RVCLB 3. (There are technical reasons why this is not possible, related to the App Stores, but I won't bore you.)
So, your main database must be called RVCareLogbookDB.rvclb and it must be saved in a particular location. Again, the previous version allowed you to select a different location, but the new version does not. I have detailed the required location of the database https://www.rvcarelogbook.com/getting-started.html
If you have not renamed the database or have not moved it, then you probably won't have any problem. But if you see no data when you launch the new app, the above is likely the main reason.
Note: before moving or renaming files, make sure the app isn't running. And always backup your files first! 😃