Is there a way to do this, or is it just the SUP way?
The custom template always reverts to Architecture units even after restoring Decimal several times and saving.
Just to clarify, it saves the units once the drawing is started and saved, then reopened. It’s when I first created the template and then create a new drawing only.
It’s not a critical issue, obviously, but a little annoying to have to change it every time I create a new drawing.
Open a template, make any changes you want to style units etc
then use Save as Template to save it for future use. Tick the box for make it your default.
make your own.
open an empty file, adjust the camera (parallel, perspective), the style, the scenes, even the content (want to see a cube instead of a person ? done) and finally the unit (basically, everything you can adjust in the “model info” panel
Then file / save as template ant tick the default template option.
edit: got box’ed 
Sorry, I guess I should have said more clearly this is a problem with the Template I created. I went through and set what I wanted, including Decimal units, but every time I reopen that template it has reverted to Architecture units. I’m not going to burst into tears if this is not something I can customize, but I might make a grumpy face for a few seconds. 
Attach the file.
…Aaaannnnd now it’s finally opening with Decimal units after opening with Architectural units 1000 times previously.
SU Forum guys have powerful magic. I salute you.