Here are some ideas. Other members will chime in with more.
1. Look at this picture. Which fuse holder do you have? If you have the one labelled NO!, it may have a bad internal connection that doesn't contact the fuse. You should replace it with the one labelled YES.
https://www.trailmanorowners.com/for...pictureid=1873
2. When you say "No 12 volt power", are you checking in the front half of the trailer? The rear half won't go live until you raise the shower wall.
3. In some model years, the converter had a master 12-volt distribution fuse. Can you tell us the make/model of your converter?
Bill