It's probably the port to ingest antifreeze into the trailer plumbing during winterization. Attach a short piece of hose, turn the valve below it to winterize, put the hose into a gallon of antifreeze, turn on the pump and it'll draw the antifreeze into the system.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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