I am slow at seeing this request since I was camping through most of the summer and fall. I only happened on your post as I was looking for some info. Hope this helps if you haven't already put yours in.
I put a shower pan in that is about 30inches by about 38 inches. It sits in the front right corner and goes across the doorway. It has a drain at both ends. Most of the time when I camp I have the head of the trailer slightly higher as I like to sleep with my head higher so the drain towards the tail of the pan is the main one I use. Sometimes though I am parked with the trailer still hooked up which makes it slightly nose low. That is what the second drain is for. Here is a link to my shower set up. viewtopic.php?f=42&t=55219&start=210
Be sure to scroll down the page to where I put a bench in.
The pan is cut from FRP , and it is holding up well. It get lots of traffic from muddy boots and gravel, but cleans up easy and doesn't seem to even get scratched. The sides are simply pine or fir that is well sealed and it was put down with silicone sealant under it and on the inside. So far it has worked excellent. The pan is not attached to the floor. The drains go through with about 4 inch tail pipes. To have it drain quickly, I sometimes do have to tilt the pan slightly to one side or the other, depending on how lazy I was when I leveled my CT. I don't camp in any campgrounds so I just use a five gallon bucket under the drain.
I use a custom made shower curtain (nylon material) that has elastic cord in the top hem. It dries quickly, but it is very low humidity here. it has some shower hooks attached in the 4 corners that slip over tiny cup hooks that are screwed into the ceiling. When not in use it hangs on my front wall.
For water I use a modified deck sprayer. I heat water either on my room heater or on my propane cook stove. It only takes a few minutes to heat up enough water to raise to temp of the sprayer water to about 110 degrees. The sprayer is very water miser friendly. It gives a nice pressure spray that quickly rinses my shampoo out. I use about a gallon or less water per shower.
I have found that having the shower here has been a good double use of space, and the pan helps to contain dirt and mud from outside. It is great to be able to come in muddy, sit down on the bench and have all of that mess stay in the shower pan where a quick rinse with the sprayer chases it all out.