Put a cinder block or some other large object on the ground beneath the coupler. Get a piece of 2x4 and stick it in one of the holes in the cinder block or against the large object you used. Hold your foot against the cinder block to keep it from moving as you use your hands on the 2x4 as a lever to push in the surge brake coupler. With a one-man bleeder kit attached to the caliper with the valve open, you only have to move the coupler back and forth while keeping a close eye on the fluid level in the coupler reservoir. I've bled my trailer brakes like this, by myself, on all of my boat trailers. Worked with all of them.
All the best,
Glenn