Welcome and yes thank you for the detailed explanation.
Refreshing in a forum that a lot of “Wut iS wRoNg??” type questions are asked.
Short answer is have you measured voltage output to your DC fuse panel and at the fan? What is it?
The battery isn’t the issue. With normal fla batts at a nominal 12.6V output fully charged and lifrpo4 at 12.8, this is a nonissue. IMO
Pulling off shore power through the converter, voltage may be/is higher, assuming here but haven’t tested, but again this is a normal scenario.
Could the fan just be defective?
What leads you to believe it’s too high of voltage?