It is possible if you have slide out one side, that side of the trailer is heavier and could cause a lean, even with the slide in. (but usually no.) The slide mechenism goes across the trailer and the weight of the slide should distribute to both sides. But depending on the way it's made, it's possible.
One tire should not make it lean. But, in order to test the theory, why not remove the spare and see if it still leans?
Did you notice it leaning before, or is this something you just now noticed? Meaning, something might have changed to cause it to lean. How is the inside loaded? Lots of items on one side and not on the other? Here again, if so, try moving things around inside and see if that makes a difference. Normally, it should not. But if your springs are worn, or broken, or something out of whack, it will continue to lean no matter what you do. So start first by simply moving things around and see if you can get it level again. Normally, trailers are not "THAT" touchy. Something could be wrong. But, there again, if a LOT of weight is on one side, that might be all it is.
Edit:
And here again, are you sure you are on a level surface? Even a slight slope can make the top of the trailer (10 feet tall) look like it's leaning a lot, when it's really not. 1/2 inch difference in the rise of the tires, could cause the top of the trailer to lean a few inches (10 feet up). That might be all there is to it?
