Richard,
I have been exactly where you are at except for the PTSD and cancer. Depression is something that other people do not understand.
My depression symptoms were exactly as you described. I was so fatigued that I spent 2-3-4-5 days in the bed too tired to get up. At times I was so tired I wet the bed because of lack of energy to get up to go to the bathroom. Some days I could just set on the edge of the bed and that's ALL the energy I had. I prayed for God to take me home. I laid in the back of a SUV while my wife drove me to a psychiatrist appointment.
Two things I would recommend:
Social Security- Get a SS lawyer that is local and specializes in Social Security. Long story short the ALJ's are in bed with the SS lawyers. The local lawyers are on a first name basis with the judges and get many more favorable and quicker decisions than an out of town or infrequent lawyer. Fire your current lawyer(s) if they are not local and get someone who is. Get a letter from your doctor that simply says you are suffering from depression and unable to work.File that letter with SS. I spent 7 years getting my SSD. One horror story after another, I did not think I was in America with all SS put me through.
Doctors- My first pyschaitrist spent 3 to 5 years trying to find the right meds. He never did. He had me on 5 anti-depressants at one time. He tried one small step at a time. Examples: I was on 5 medicines. He took me from 5 to 4 anti-depressants and the next appointment in three months. I suffered for 3 months,til the next appointment. That time he changed one anti-depressant from a morning dosage to an evening dosage. And on and on it went for 3-4 years. Demand that your psychiatrist make drastic changes if things are not working. They want to ramp you up on a medicine and ramp you off with each requiring a month. I finally found a new doctor. He prescribed Pristque. In less than 24 hours I could feel a big difference.
Now I get out of bed everyday. My mornings are slow but I get more energy during the day. If I do a sizable amount of work I pay for it the next day. I require much more sleep than before, about 11-12 hours a night. I can work 5-6 hours a day but not like before.
Hang in there I know some of what you are going through.
Father ,I pray for Richard that relief may come his way quickly.