Cheryl, (Sutton, MA)
The only cure is to replace that section of ductwork. If your ducts are routed on top of the flooring inside the RV this task is relatively simple. If, however, you have a central distribution ducting system, it becomes more problematic. In that case, first try stuffing crushed-up black & white newspaper pages into each duct as far as you can easily reach and then closing or taping off the ducts for two to three days. Every two or three days, remove and trash those pages and stuff new crumpled pages in there. Repeat as often as necessary. Crushed newsprint has removed fouled food odors from refrigerators in the past, perhaps it can work wonders for mice urine as well. Check with your local pet store also; they may have urine odor products available.
Wish I had better news for you, but animal urine is definitely one of the most difficult odors to eliminate. Maybe some of our members and readers have further ideas. I’ll be happy to pass on any worthwhile suggestions to you.
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