During our last visit to the BH, I decided to add some water to our never-used tank...just for toilet flushing during a power failure. I got about 15 gallons in before it started flowing back out the port. It's not the vent hose, but the fill hose. It had a dip behind the UDC and as the droop filled with water, it drooped lower until it stopped flowing to the tank. I reached behind the UDC (home-made access door), lifted the hose, and it continued to fill. I didn't have a sturdy enough section of extra hose on hand to stick in past the droop and the clear plastic hose I had got blocked at a bend further into it.
Just another thing to check when filling the tank. I'm bringing some short water hose I've got this next trip to make a better fill tube extension. Might even do some reno on the fill tube while it's in storage next month.