Do you know if yours has/had a grey tank? Since it is a '93, I'm thinking that it might. If that is the case, I can't help you with a picture; however, if yours was plumbed without a tank then I might be able to get some pictures for you.
Just thinking out loud here ... since yours is a '93, I'm guessing it is pretty low to the ground and the portion that is missing might be from the shower drain over. Possibly including the part that connects to the vent pipe. I say this only because I hit a bump before mine was raised and lost that entire portion. For what it's worth, it's pretty easy to fix. Just make sure to use the black pipe (ABS) AND the glue that is appropriate for it.
If you can, a picture posted here might allow us to help if another '93 Casita
owner doesn't step forward.