Ours came with a holder, it in a box under the sink. The bathroom vanity has 2 doors, one right next to the toilet, (sun seeker 2401W), I read someone here turned the door over, so it now opens in front of you when you are sitting, and put the holder inside the door.
My goodness, I thought it was so clever, but I can't even explain it. Anyway, ours sits on the counter til we drive. I like the plunger idea myself.
Full time 2016 Sunseeker 2400WS towing a 2018 Subaru Forester.