Hey, welcome to the community!
The client library remoteStorage.js supports connecting to all of remoteStorage, Dropbox and Google Drive (with some caveats for the latter two): https://github.com/remotestorage/remotestorage.js/
The library documentation has a section on how to set up Dropbox and Google drive as additional options: https://remotestoragejs.readthedocs.io/en/latest/getting-started/dropbox-and-google-drive.html
An example in the wild would be Litewrite: https://litewrite.net/ (source code)
If you have any questions and/or run into any roadblocks along the way, just post here in the forums and we'll try to help.