Several thoughts. One, start with grid paper and proportion the basic shape accurately. Your drawing is longer than the actual shell, so it looks like things will work that really won't. For a 13' Boler, figure 118" long and 77" wide with rounded corners (radius around 16" on the back corners and 24" in front, rough guess). The door is 26" from the front of the trailer and 20" wide. Wheel wells (which will preclude your intended bath location) are just aft of the door. I'd guess they are around 10"x16"x10" high. Ideally you will want to measure and mark where the floor is high and where it's low, but for that you really need to have the trailer.
Second, start with the bed. That's the most important part of the trailer. The stock bed is only 45" wide. If that works, fine, but most people would want at least 54". To create more space you could consider an asymmetric bed that's wider at the head and narrower at the feet.
Third, make sure you consider where you will put any intended mechanical systems. Remember that a 3-way fridge
must have exterior vents, which will need to be on the sides of the trailer. If you're going to use a DC fridge
, them you can be more flexible.
Fourth, consider weight
distribution. A front galley can tend to become very tongue heavy. If you're going to have a fridge
, nearer the middle is better.