Re: Unknown cause of severe fatigue/muscle loss for the past 4 years. Symptoms started immediately after dramatic diet change, but have not gone away

The detail you have will be very helpful to someone who has a deep understanding of our biochemistry. Someone will get to the bottom of your problem.

Did you get a copy of the test results from the ER? Never leave without a copy. Don't fully trust the interpretation of the mainstream doctor. Sometimes significant leads can be gleaned from the tests even if the levels are not terrible.

Was an iron panel part of the tests?

Have you ever done any test for low stomach acid? You need adequate amounts of it to absorb protein, minerals and so much more.
https://www.livestrong.com/article/493568-how-to-test-for-low-hcl-acid-in-the-stomach/

When you switched to the vegan diet, were you careful to eat the rare foods you need to get a full balance of amino acids? I hear this requires some planning.

Have you tried to go back to the Paleo type of diet, especially along with support for stomach acid--apple cider vinegar and or betaine HCL--plus digestive enzymes and maybe OxBile or similar? It might take over a month to notice changes.

Have you ever considered using a diet supposedly consistent with your blood type?

Anything else happen around the time you switched the diet? For xample maybe you received the flu vaccine containing 25mcg organic mercury, especially if you received several vaccines on the same day. People in the military are especially vulnerable but the following review of VAERS data shows that some kids are also.
https://worldmercuryproject.org/news/anthrax-vaccine-safety-concerns/
http://vaccinesafetycommission.org/pdfs/04-2016-JPANDS-Miller-Vaccines.pdf Tweet