Check your water tank vent
When I was @ Michigan Int'l Spwy in June, I had the water spitting back out the filler opening. I thought it was due to the high pressure of their water system. I was only able to fill @ a very slow rate. I stupidly assumed the tank was full (never checked the actual level in tank). Even w/ minimal use over the weekend we were nearly empty Sun' night.
Fast forward to my trip to northern Ontario last week. I noticed that there wasn't any "spitback" thru the vent. I popped the vent screen out w/ a knife & saw that the back side of the screen was covered w/ scum.
After removing the scum, I could hear air rushing out of the vent & the water stopped sloshing back out of the filler until the tank was actually full.
I've never seen or heard any mention of cleaning the vent screen, but will check it on a regular basis.