[JRTJH]

A hypothetical: What would happen if the neighbor sharing the "Y" connector left his black tank flush hose connected to the "Y" and his black tank flush back flush valve wasn't working? Would/could the OP turning on water in his trailer cause a "venturi effect" at his part of the "Y", pulling contents of his neighbors black tank flush hose into his water supply?????

I would prefer not sharing a "Y" connector if possible, but if forced to do so, I'd have a backflow on both sides of the "Y" output to protect both supply hoses from each other. YMMV