Hi jer9 your diminishing fire after a few days is probably an accumulation of ash on the grates.. the fire box has a 'v'shape to the firebrick,, this 'V' shape acts like a funnel to concentrate the ash onto the grate, and causing the grate to clogg up and have little or no air passageways.. If you look at the underside of the grate you will probably see lots of dark spots and only a few areas of red coals .. You should see all gaps in the grates showing red coals above..
Make yourself a poker, like the photo in the post several above this post.. use the poker to go into the gaps in the grates and aggitate the ash and get it to break up and fall down , then shake the fire again, you need to get the ash off the grate and into the ashpan.. Once you get the air passageways open again, then you should have more heat..
You may just have to clean the firebox out every week and start fresh,, this is what I had to do,, I kept getting sheets of clinkers,, melted together ash that would not break up and go down into the ash pan,, so I just let the fire go out every week, shoveled out the firebox and the clinkers and started over again..
The HotBlast firebox design works well withwood, but not very well with coal..because of the above issues.. a good coal firebox does not have tapered,
V' shaped firebricks.. and the grates are over the entire bottom of firebox..
Hope this helps