From my experience with a 74 motorhome:
There is a nut and a toothed washer on each screw, nuts and bolts were steel, and fairly rusty. The nuts were very tight against the door frame and were covered with sealer, I had to dig them out. The vertical rivet lines should tell you how close the next rib is, mine only had about 5/8" working space between the frame and rib.
These are some of the screws I hoped would break but they never did. Soaked them with WD40 for a couple of days. Mine were too tight for a socket, some were so tight the frame held them for the first few turns. I considered bending a wrench but there is not enough room even then. There was just enough room to get an open end wrench at an angle and barely grab the nut.