I also have mctd and had a very extreme case. My symptoms were different but every case is unique and there is a wide range of how the disease progresses as well as how it responds to treatment.
The first order of business would be to get you to a specialist clinic like Johns Hopkins which has a Myostis Clinic (the only one I know devoted to these diseases in the USA). They have experience dealing with MCTD and can help to confirm the diagnosis and recommend a course of treatment and possible physical therapy to help restore weakened muscles and help with your stamina.
Fatigue is a difficult issue and can be caused by many things. My guess is that the doctors will tell you the first thing is to get your immune system under control.
I have come back from losing 40 lbs of muscle mass and not being able to swallow. Spent several months in hospitals and doing out patient therapy and have continued on my own for more than 2 years. I have progressed from being a quadriplegic to returning to golf, skiing, tai chi...and lots more. Fatigue and weakness is still with me but much improved after all the work. This is a complex subject but I am an example that recovery is possible even in extreme cases.
There is another site, www.myositis.org with forums and more patients with mctd. There are people there who have gone to JH or can recommend other doctors/clinics in your area.
Contact JH and start the process. You do not want to leave it untreated. Unfortunatley there is no magic pill for fatigue but once you get things under control therapy can address that issue to some degree.