If it is leaking fresh water I would check thru the toilet paper door to see all the plumbing under the sink. This is also where the water line to the toilet comes from. All the plumbing for the outside shower is there too.
The dump valve access area is small but that is where your low point drains are.
Taking the bottom of the camper off is another way but not easy in the back due to the side stairs, light wiring, dump valve, and the sewer hose.
If your leak is fresh water you should be able to find it under the sink thru the TP access door.
Not a lot of good access on that corner due to the side ladder and fridge but all the plumbing for that bathroom is under the sink. I wonder how hard it would be to take the whole fiberglass corner sink out?