Looks due to way off-camber. Maybe sagging springs, overloaded trailer or an unbalanced load could be causes but you say you've never had a toy in it. Ever been to a scale all fully loaded up tho? What is age (per date code) and load rating of tires? If original tires and 7-8 years old or more, time to replace all 4 anyway. I see that the 19RR toy hauler has about 4K lbs CCC on a GVWR of about 7500 lbs so it would take a lot to overload it without a toy in it. If you do go to a scale, try to get side-side and axle-axle wts or each wheel.
As noted, underside of the trailer looks somewhat in need of TLC. Have the wet bolts ever been lubed? Equalizers okay?
I would spend the couple hundred buck it takes for an inspection at a frame and axle shop and get a detailed inspection. Could be any number of things or a combination of them, some of which aren't always obvious.
Don't forget to post the outcome!