Luggage,
to the forum. Here's hoping you find lots of information to help you maintain your Montana. Here's "also hoping" that you leave some of your experiences with maintenance on your rig to benefit others who might be facing challenges similar to the ones you've managed to "outsmart"
We look forward to hearing from you often
Now, as for your door. Yes there is a 3/8 or 5/16 nut inside that little hole. If you remember back to Grandma's screen door, the one with the diagonal wire with the turnbuckle in the middle, the one that was used to square the door, well there's something like that inside the door on your RV. Chances are, that after 10+ years, the nut is rusted onto the threaded section and you'll break the threads if you're not careful. A little squirt of thread lubricant or a similar penetrating oil a day ahead of when you plan to work on it might help, but be careful with twisting too hard or you may wind up damaging the door. As for being out of square, that adjustment doesn't just "happen to fail" this far out from installation of the door, so my guess would be that you've got a tension rod that's either rusted in half or has broken at the top or bottom attachment. This would allow your door to droop out of square.
You might try adjusting the door, but if it won't adjust, you may, when the weather gets warmer, take it off the hinges, disassemble it by removing the multiple screws around the perimeter, pulling the inside panel and taking a look at what's broken or rusted that's allowing your door to sag. I had a similar problem on a pop-up truck camper and had to completely rebuild the door. It wasn't difficult, just time consuming.
Good luck.