What I find that helps is to open the door and twist the knob counter clockwise at least 4 or 5 full revolutions. The knob is basically a bolt. The inner latch is a nut on the bolt. While you have the door open watch it as you turn it counter clockwise. It extends longer on the bolt. Over time,,,, You can get it too tight and it misses the RV side wall and won't keep the door closed.
By rotating counter clockwise several times you extend the latch out far enough to be in range to start working again. When you close it this time it will rotate 90 degrees into position and then tighten down on the bolt and catch.
Larry