I just checked the shell and inside door surfaces with a long bevel guage: my doors are sprung out on the forward edge! So I’m with Fritz: keep adding lumber till it all fits: first attempt will be that 3/4″ x 1″ trailer bunk stiffener up against the forward window sill and lapping 5 1/2″ onto the existing door bottom stiffener almost to the bottom of the door. Β I’ll do the same cut in the existing stiffener that you did except perhaps deeper into the fillet. I think you will find that the glass over your trial cut won’t be enough , probably you’ll have to add lumber. Feel free to disagree/correct me.

My camper won’t leak and is good enough to take on the road, but I would like to get my door fit better than it is so far. Β Gonna have to sand some varnish off and add lumber.