FWIW, when I level my trailer at a campsite, I make sure the shower drain is downhill, in my case, that means I set it slightly lower on the curbside, and slightly high on the nose jack. its not enough to upset sleeping on the bed. it also helps the kitchen sink drain completely.
Funny? story: when I got my trailer (a 2014 E21 classic), the kitchen sink always had water puddling in it, and I realized the sink flange was higher than the floor of the sink. So I removed the drain pipe, and disassembled the drain flange etc, went by my trusty old local hardware store and was looking at the plumbing stuffs... old store guy comes by, sees me looking at the putty, says don't use that stuff. I explain my problem, he goes OH, thats the one thing its specifically for. put a good bead of it under the flange, the washer goes UNDER the sink, clamp the assembly hard to squeeze out the putty, wipe clean with a damp rag, and have a beer!
worked great. sink now drains dry.
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