How to change a window crank
There are other ways to change a crank, but we found this to be easiest. It involves removing the screen frame and cutting into the window frame. It is very simple to do and takes about 10 minutes in total. Other methods are far more involved.

Removal: You need to make 2 cuts on either side of the crank using a hacksaw. Then bend the frame back with a pair of pliers. Undo the mounting screws and pull out the window crank (gear box). You will then need to take the crank to an RV shop where they can match it to a new one.
Installation: Do the reverse!

After you bend the frame back, you can solder the cuts (difficult with aluminum?); however, this isn't really necessary because once you put the screen back in place, the screen frame will cover the cut marks.

Alternate way of removing the cranks

This was contributed by Chris Demerah:

To change the window cranks I had to remove the windows from the the trailer and disassemble them. There are about 20+ screws that hold the windows in. The screws go from the outside through the aluminum window frame, then through the fibreglass and into a wooden frame on the inside. The wooden frame is covered with aluminum on the inside. So, you have to pop off the black plastic strips on the outside of window (mine were mostly degraded from the sun) and simply remove the screws.  Once the window is out, there are eight screws that hold the frame together, I took these out (spray some WD40 or penetrating oil on them). There was a couple of snap rings holding the shaft on, you have to take those off.
 
I tried to get the gear box out without all this dis assembly but I could not see how to do it. (I don't think you can). I found the matching gear box at an RV parts shop. I also bought a roll of the putty tape that is used to seal the windows and the roof vents.
 
I ended up replacing the wood that the screws bite into because it was rotten and I used Stainless Steel screws to put the window back in.