Take a look at that frame and notice the "missing" structure where the door on the left side interrupts the frame. This is one area that needs reinforcement as there is a lot of flex here and that twist caused the frame problems in the bends on both sides and where they tie into the structure.
If it were mine I would also box in the already caves in sections on the inside of those bends as the tube has already failed there or they would still be straight.
Depending on what you are planning under the floor I would also install a cross beam from the newly boxed bends to each other.
By the way the idea of a fish plate would include a longer taper and a sideways vee instead of the rounded plate ends as they still will tend to concentrate the stress in the end of that circle. Best would be a Vee with the weld extended an incl or so past the end of the plate.
For the door frame area I would put two 1 1/2" X 2" tubes flat across the opening and extend them far enough to transfer the load to the front and rear with bracing to also reinforce that connection as this is also an area that cracking takes place.
I added the tubing and a bracket on my 16' Scamp
. Here is a picture of the doubled cross member and the bracket forward of the door.
The view of the other side with the added section under the frame forward of the door. The extra 1 1/2" square tube is the end of the structure added for the front bathroom