First off remove the door completely when doing this project. I did the latch first with just barely enough room to work and that was with the side, top and bottom trim pieces removed. The door brackets hold the skin tight. When I insulated the door (as you probably will too) I removed the door and wished I had when doing the latch.
The latch will be installed from the outside in. You will have to cut a hole in the outside skin (hole only needs to be large enough for the assembly to fit through) after determining location (Location is totally up to you) and trim some wood from the inside frame support to allow for this to set flush. You should be able to figure that out from the photo's
I removed the trim pieces of the door and pulled back the luan and tin after removing about a thousand staples. I put the latch end flush with the frame of the door. You will have to use a rasp or something to remove some wood for this to fit but it's easy especially if you remove the door from the trailer. I also made a framework around the latch for support when securing the two halves together because there is nothing there for support.
Here is the door without the skin and showing the support built around the latch location.
I would get the latch in even if you don't have the correct depth. Get an idea as to where the hole needs to be cut in the outer trim piece and make get that hole cut. Once that is done you can put the outer trim piece in place and work on the depth portion. I made mine flush with the outer trim when complete. My dads new trailer has a trim piece that covers over the door opening where as mine don't so this is only what I have done here. I would think it's pretty much the same though.
Now, when your done with that installation, I would recommend going to a trailer shop and picking up a latch bucket for a couple of bucks. My dad had his latch installed when they made his trailer but they didn't include this bucket. All the trailers with latches at the local trailer shop I use have them already installed.
Figure out where the hole nees to be and cut it out so the bucket will fit.
Once that is done, add some silicone and it's finished.
Now wasn't that easy?