This is a tricky one, but there is a solution. First, there is suppliers that provide trailer replacement parts and if you find the website for the manufacture of your particular trailer, they will sell you replacement pieces.
Now, if you need an affordable different solution, another idea. I don't know if you are talking about a tub/shower combo or shower unit only but if it is just a shower unit, here is what I would do. You can go to a plumbing supply company (city plumbing and electrical comes to mind here in the Atlanta Area) and purchase a fiberglass shower base. You can get them in various sizes, 30x30 up to 36x60 depending on your need and the amount of room you have. You can remove the existing unit and replace it with the new base and build from there. Ok, next, for the wall area you can go to Depot or Lowes and purchase 4x8 FRP (fiberglass reinforced panels) these can be cut to the size you need. You will also need to purchase inside corner pieces and glue. You can use a shower rod and curtain in this type of installation if need be,see my article on how to build your own http://hubpages.com/living/How-to-build-a-ceilng-m... but it is also possible to re use the existing shower door you have with a few modifications.
You should be able to complete the entire project for less than $250.00 if you do the labor your self. The other thing is you may need to replace the drain strainer to a standard size for the new pan, check with the plumbing supply house to see if the hole size is the same. If not you will have to buy an adapter while at the specialty plumbing store. To make the connection, here is another little trick. When you remove the old unit, cut the 2" drain pipe a couple of inches below floor level, add the adapter with pvc glue and let dry. Next install the drain strainer on the new fiberglass unit and hand tighten, test fit the unit in the area first, then add glue, reset and let glue dry and your done. It sounds pretty easy, but I warn you, it takes time and you need to test fit all fittings before you add glue, if the cut hight is wrong it will either leak or not sit down on the floor properly.
Good Luck and I hope this helps.