As a temporary fix, you could install a Fernco coupling as you suggest. Any decent hardware/plumbing store will have them for a few bucks. They come in different sizes and look like this:
For a permanent fix, since it sounds like there is an old section of pipe still in the elbow that does not protude from the elbow, you are probably going to have to cut the elbow off and install a new one. You'll probably need to install one or two pipe unions to make up for the lost length.
You may actually find that the Fernco works quite well. I've seen them on drains as a permanent install. Not ideal, but they work. You could always leave it on there permanently until it fails, even carry another one in case it fails on a campout.
You are lucky the pipe didn't break off at the tank.
Dave