I have a sewer connection at home, so I've used such a dump station only a couple times in the decade we've owned an RV. But I quite understand your pique. I've encountered plenty of self-absorbed inconsiderate entitled boobs many times in other lines. And there have been a few times when it was all I could do to not unload on one of them.
But I do remember once, many many years ago, pulling into a busy gas station to tank up and finding myself behind a guy who tanked up, went inside, and evidently got a hot dog, took a dump, and chatted up the pretty young thing at the register until she finally kicked him out. By then, having waited about 15 minutes behind him at the pump (AFTER he'd filled up), I had gone around him, backed up within inches of his bumper, and started pumping, when he finally staggered out and was highly offended that I had him blocked in. I remember telling him I'd waited on him for 15 minutes, now it was his turn to wait on me, and I'd be done as soon as the tank fills and I go in to pay for it. No, he wasn't happy about it.