Best photos I have of the vent hose and why it spits when you fill it and siphons water out as you drive. The photos show why you also can't get a full tank of water if you fill at a water filling station while you are hooked up.
The last photo is not my camper but shows the effect of an out of level fill attempt.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW