That's a very general question and I believe you'd get a better response if you gave a few more details. Is the smell concentrated in one particular area (bathroom, kitchen, bedroom)? Does it smell like rotten eggs or like sewer? Did it just start recently or has it been going on a while? Is your trailer new or did you just purchase it used from an individual?
A bit more information would help us all try to help you diagnose and identify the issue.