The washer SHOULD NOT be plumbed to the black tank drain for the reason the OP mentioned, possible overflow. Sure, it's a great idea getting that soapy water to flush out your black tank valve and poop, but one forgetful moment not opening the black valve and my, oh my. You'd have to trash the trailer. Furthermore, I don't see how a manufacturer could run a washing machine drain directly to the sewer line without first entering a holding tank. That would eliminate your chances of ever being able to do a wash when dry camping....unless neighbors didn't mind you dumping on the ground.