Your leaf springs on that side are probably weak. Check to see if all spring stacks have the same arc. If not, a I would replace all springs since the others will flatten out under the weight and the strain of traveling. Also, check the spring holder if it's bent in. This will drop your trailer about 2".
I had the same problem except mine leaned to the right. What I found is the springs are giving out and not holding the camper box up like they are supposed to. I bought new springs and had a local change out all four springs, new shackles and was good to go.. No more leaning.
If you crawl under you can actually see the springs flattened out with no cup upwards.