A lot of tag axles have self-steering joints on them. Many years ago, I modified a class C mini motorhome for someone. It also had a self-steering tag axle. I changed the Toyota 4 cyl, for a 400 small block Chevy, and put air ride on both the drive and tag axles. I had in cab controls for the air ride. It was a fun thing to drive. If you ran the air up on the drive and down on the tag, you could pull wheelies at stop lights. Or run the air up on the tag and down on the drive and do burnouts as long as you want. It always surprised people because it looked basically stock.