The filler cap on my Komfort is probably 3' above the tank, with the vent right next to it. If I fill slowly, I get no siphoning back, especially if I rock the trailer just a hair to get all the air out that I can. I've never had a water-loss issue and would have complained to my dealer PDQ if I had. My b-i-l rented a Cruise America C class and it was delivered full, but the CA people forgot to put the cap on the filler, so he lost most of his fresh water on the way to our camp site. As there was an additional issue with the water system (like a broken fresh water line under the sink), he was forced to drive on to Flagstaff to get that fixed. CA did refill his water tank and reimbursed him for the gas for the near-100-mile round trip to Flag and back.
Lyle