For the purpose of benefits, it shouldn't matter whether ME/CFS is classified as a mental condition or a physical condition. Quite a few mental conditions, including severe depression, cause severe tiredness, so even if a particular doctor thinks your CFS is a mental condition, this should not be a reason for ignoring the reality of your fatigue or its impact on you. To give an example, I have bipolar disorder and most nights these days I sleep 12-13 hours, and I also rest during the day. That's a mental condition. So if someone tries to say you can't be tired because they think your CFS is a mental condition, tell them it isn't relevant - you're fatigued.