Jamming a garden hose end in the filler port can lead to disaster. People have blown the flex filler hose off the tank or the filler port (thats me) or swell the tank to such a size as to bend the mounting brackets and have the tank wanting to fall out of the RV. All kinds of problems can ensue. The small air vent next to the filler port is designed to admit air to the tank to replace the water the pump is drawing out of the tank, it is NOT designed to vent air out of the tank when you have a garden hose running full blast into the tank.
I discovered, after the fact, that the owners manual for my trailer (skimpy as it is) actually instructs you to use a reduced filler hose that slides into the fill port and allows air to escape around it. Camco and others make such a filler
For me, my filler neck hose was cut by the factory way too short, and was unsupported, so the hose had apparently come off before (evidence by my finding an offset screwdriver wedged behind the wire harness adjacent to the filler port on the inside). Anyhow, I replaced the filler hose and strapped it and a new vinyl vent hose (old one had mold in it) up, and now I use a very long adapter I made that reaches all the way to the tank. Granted, this is a unusual situation but these adapters are made for a reason, and that is to keep from damaging the RV.
60 psi can do quite a bit of damage.
EDIT: NY_Dutch said the same thing earlier...............
A Bigfoot 25 ft trailer owner swelled up his tank and destroyed his tank supports (by shoving the hose into the fill port). It turned into quite the mess to repair as the tank is inside a fiberglass shroud that is sealed to the frame all around the edges (4 season trailer). My fresh water is inside and the waste tanks have a similar shroud covering them.
First pic shows the final filler hose install. The vent line is mostly hidden behind it, strapped up with it. The second pic shows the filler I made to reach all the way to the tank. Third pic shows the (arrow) mold inside the old vent line. In removing this vent line the nipple on the outside filler broke off and I had to replace the entire filler adapter and door.
Charles