CFS to seems like a catch all when docs don't know what else to do.

Since gut and diet can have a huge effect on fatigue and docs usually don't ask about what you eat I would rule that out first. You can do this yourself by changing what you eat or limiting to certain foods for a few days and see what happens. In fact I would try to see if you feel better if you fast for just one day and drink only water, no sugar drinks.