I got around this by just simply having roundcube in one folder for webmail.museumtour.com, and then having another roundcube folder for my other webmail accounts, that for example, webmail.example.com would go to its own webmail that only has the configuration for example.com and webmail.museumtour.com only has the configuration for museumtour.com. This is by far a more simpler way to host multiple domains using the same webmail. Comon how long does it really take to install roundcube, it tooke me all of about 30 minutes to get everything working.