I finally got the sewer compartment door opened.
As it turns out it is a two-person job. I had to enlist the services of my lovely wife.
Here's how we did it: I pried, and held, the bottom steel plate toward the center of the coach using a large broad-blade screw driver. Then, while my wife pushed against the door at the latch, I pried, and held, the lock plunger ("bolt") straight up with a smaller screw driver. While holding both screw drivers in place, my wife, then, with all her might, pulled the door open. Whew!
Now, my wife stresses that one needs to wear gloves when pulling on the bottom of the door because it is sharp and her delicate hands needed the protection.
Now, after the rain has stopped, I'll remove the latch mechanism, tear it down, and see if I can repair it. If not, I have a spare.
But, the key, again, is having someone push and pull the door at the right times.