The o-ring are accessed exactly as the diagram shows. Removing the core plug and pin & spring allows you to get the larger o-ring. Once the pin is out, you can remove the little brass washer and the little o-ring which is behind the brass washer.
I don’t expect a tight fit-up of the hose connector. Unless it is exceedingly loose, it is normal. If it is exceedingly loose, it is a worn-out or incorrect hose connector, not the tank fitting.