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There is absolutely nothing you can put down to remove the smell of cat urine and feces - simply because they are going to continue to use the area. If you have neighbors, undoubtedly they will begin complaining as this is extremely unsanitary. You need to get all of the cats out from under your trailer home and prevent entry. Ideally, contacting your local rescue agency will be the best for them as they will spay & neuter them and get them all good homes. You are basically sitting on open, raw sewage and the only solution is to prevent the cats from getting to the area or having them removed. Once they are gone, you can hose out the area to remove the smell. I would mix Zero Odor in a spray system that you attach to a hose, and spray the area thoroughly. This is the best product on the market - www.zeroodorstore.com.
Please let me know if I can help further.