As a mechanical engineer with some weld design and stress analysis experence I will throw in my thoughts on the subject.
The thickness of the rectangular tube is only 1/8 thick, based on measurement of the thickness of the similar rectangular tube used to support the slideout. The bracket for the pin looks to be at least 3/8 thick. As a rule you only need to make the weld bead as big as the thinnest member (1/8 inch) for full strength. The welder in this case made a very large weld which looks stronger, but likely weakened the structure by overheating the rectangular tube section when trying to apply enough heat for the thick bracket, and maybe even thinned out the wall of the rectangular tube in the process, a common occurance called burning.
The frame normally experiences compressive forces on the top of the tube and tensile stresses on the bottom of the tube with the center of the tube being neutral (neutral axis) when the trailer is not under acceleration or deceleration. Steel is weaker in tension than compression as most materials are. The farther from the neutral axis the greater the stress. So the bottom is going to have the highest stress which is apparent from the failure picture.
Since the modification puts the forces on the hinge pin area, at the corner, where the crack started is the most vulnerable. Instead of being distributed evenly, as in the standard trailer tongue, the forces are transfered over to the side of the hinge pin. The hinge pin necessarily is not perfectly ridgid in order to provide the swinge action. So the vibration of traveling down the road caused a fatigue of the metal on that corner and caused failure.
This is not the kind of failure you would expect from folding the trailer hitch backward to the parking position. These stresses would be very small compared to the stresses experienced when driving down a road with dips and bumps especially at typical driving speed. These irregular surfaces in the road will cause the load to shift significantly and rapidly up and down causing bending stresses as well as longitudinal jerks tensile stresses on the tow vehicle hitch. Those combined forces, vertical bouncing and horizontal pulling, fatigued the metal and lowered the tensile strength to failure.
It is an easy repair for a certified welder. First remove the electrical cable attached to the bottom of the rectangular tube. The paint and rust needs to be removed and a v-notch cut in the crack with a hand grinder. He would be advised to support the frame by placing a jack near the crack under the rectangular tube on the trailer side of the crack. This will tend to pull the crack together. Weld the crack all around to the other side. Also weld the crack that appears to be forming on the top bracket that shows corrosion.
A patch could be welded over the crack for added measure of security but not totally necessary. If using a 1/8 thich steel plate as a patch grind away the weld flush to make a flat surface.
Then grind flat on the bottom surface of the rectangular tube. It recommended to weld a 1/4 inch thick strip as wide as the rectangular tube on the bottom side of the rectangular tube at least a foot long. But a strip the full length of the rectangular tube to the trailer frame would be better. Welds would be made completely under the hinge bracket (with the pin) and cracked area. Then welded on a spacing of 2 inch long every 6 inches to the end of the strip. The strip should be welded on both sides of the strip along the rectangular tube. Another 1/4 strip on the top side of the rectangular tube would add even more strength, but not necessary. Be sure to clean, prime and repaint the bare metal surfaces.
Hopefully the Trailmanor Factory has some experience with this failure and would give a good repair recommendation also.
Good Luck,
Bob W.
