Unfortunately, things like that happen everywhere. Personally, I don't use Flying J, get my propane filled up at my nearest RV dealership or Tractor Supply Company if they are not open, and get my diesel at normal auto gas stations, even towing my 40 foot 5er.
And yes, many, many times I've had a gasoline operated vehicle pull into the only bay that had diesel ahead of me, even while trying to manuver trailer into the spot. Then they jump out and run inside and I'm now in a situation I can't go forward and I can't go backwards and just have to wait.
Sometimes, I've even seen them beet me to the pump (and there were other open bays), jump out, run inside, take 15 minutes, come out and leave, never getting fuel.
It's only getting worse too.