We finished out my horse trailer and it has an 8 ft short wall. I'll try to describe what mine is like the best I can. On the long wall, I have a sink and 4 foot counter (with a fridge built in), the door, and a toilet/shower combo. Being you're short on space, I would strongly recommend the combo system. On the short wall, I have a closet, and futon. Providing your door is on the long wall, you could probably have a similar setup with just a smaller counter (possibly taking out the fridge) and no closet.
As far as tanks go, we have 3 tanks under the trailer. 1 for clean water, 1 for the shower and sink, 1 black tank for the toilet. The hot water heater was a bit tricky to get in. You have to cut a rather significant sized hole in the side of the trailer (it's a scary moment) right under the sink. It's around a 12 inch by 12 inch size hole. Don't worry, they come with a nice pretty cover for the outside of the trailer. It takes a bit of planning prior so you know how you'll run your water lines. We ran ours through the floor (you'll want to build up the floor about 2 inches). The water pump we put right next to the hot water heater. The trickiest part was the electrical for everything. We have the fuse box work as a step to get up to the gooseneck.
Hope some of this is helpful to you. I can go further in depth on anything if you need me to. Good luck! It's a fun project, but be prepared to get frustrated!