Wow your doctor was so unhelpful!
Fatigue, sweats, joint/muscle pain, hair loss, low mood, weight gain are all symptoms of low Vitamin D (plus others).
When I was diagnosed as deficient I found eleven other people I knew were also diagnosed.
Current thinking is that supplementing should be done, especially throughout winter months.
Having an ‘optimal' vitamin D level has a lot of protective health qualities too.
There is lots of science/research based information to read up on using the internet, maybe print some off to help you start another conversation with your doctor.
If you do supplement, as the doctor suggested, then you really should have your levels monitored so I don't really understand why they don't want to get a starting level.
Hope you get some help soon