It's very weird.... Have you crawled under your trailer and poked around looking for a cap or "glob of spray foam" ??? Most trailers from that era had a direct 1/2" drain line with a cap on it, usually extending 2-3 inches below the bottom liner.
If you're not the original owner, some previous owner may have damaged the drain line, elected to cap it and seal over it. You'll probably find the 'bump" under the bottom liner if that's the situation.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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