Holding tank location
Your toilet should be pumping into the TOP of the holding tank. and your holding tank discharge line should run out of the bottom fitting to a deck pumpout or overboard discharge. The only downward pressure would be moving the slug of waste from the toilet to the inlet of the tank. Once in the tank, it would not go back in the hose unless grossly overfilled.
If it were me, I would ensure that clean water was pumped in the hose section after toilet use I would not count on the joker valve holding back a slug of sewage that would continually try top get back in the toilet.