Most tankers have multiple compartments.  Simply don't fill all of the compartments to keep the load from sloshing.   If you want to fine-tune the size, put a combine innnertube in the compartment and air it up however much it takes to make the space right for what you need to haul. 

A friend hauls most of the 10-34 in this immediate area, and he has had an aluminum trailer for maybe ten years now.  He doesn't leave it sitting around with fertilizer in it, but it's worked fine for him.