In my first trailer I went with the wet room approach.. I had a toilet, small sink and a drain in the floor. The room was 100% covered with FRP. Its plastic sheeting you can get from any home improvement store.. Same stuff thats in gas station bathrooms. Although it worked very well and saved space. We did not like it because the trailer was only two rooms.. The wet room and the everything else room. Once you took a shower in there it was hard to get dressed with out getting your cloths wet.. It also ment if someone went to use the toilet right after a shower the seat and lid was wet.. I know your not talking full bathroom so those problems won't matter.
The shower in my current trailer is just a rv shower pan (24"x32" I think) and 3 walls that are lined with FRP. Then we have a toilet and closet.. So the space works like typical bathroom in a house.. Only difference is now there is no sink in the bathroom.. We have one sink and its in the kitchen area.. It saves space and we found in the first trailer one of us would need the sink often while the shower was being used or toilet.. So now with it in the other room solves that problem. I did still line the entire bathroom in FRP though just because it makes the room very easy to clean.
Some of the shower ideas are pretty creative. Both my trailers just had water tanks with an RV 12v pump just like in a regular camper.. Its really not that hard to do.. Or expensive for that matter.. The pump and the tank will cost you a 100 bucks.. I myself have to have a water heater cause we camp in the winter months also. But if you dont' need hot water its pretty cheap to setup a pump and shower head..
Bathroom in my old cargo trailer


New bathroom


Course you can tell the new bathroom is a little bigger. New trailer is 7' wide and the bathroom is about 36" front to back.. The old trailer was only 6' wide and the bathroom only was about a 48"x36