Just to answer part of your question, I am one of the people that writes for RV Park Reviews. I also write for Trip Advisor and a few others.
It is obvious that you ran into a person that does not know much. This happens to us a lot. It is also one of the reasons that we have started to avoid using crampgrounds when we don't "need" them. Some are wonderful and helpful and then there are the rest.
I can also tell you that if they fill the "Big Ones" they have a meter that reads in gallons and probably have the fitting to fill your 20# can. Most do. In actual fact, I have never seen one that did not.
Matt