Oh, I got it. Ours is hard to fill also because it goes straight into the tank, not down to the tank as you would think. I haven't seen any check valve or anything but the only way to fill is to have enough pressure to get the water really flowing into the port.
When filling from a hose I use a plastic spout about 8" long. Screws onto the hose and sticks way into the port. Really gets the water moving. I bought it at Walmart years ago.
When dry camping I refill ours with a 6 gal container and it does alright until the very end when I get a lot of backflow. It has about a 6" spout. It makes you feel bad when you first have to pump the water into the container then carry it to the RV and heft it up to pour only to have some of it pour out on your feet at the very end.
2010 Lexington 255 GTS
2006 Allegro Bay 34XB FRED sold
2000 Jeep Cherokee Toad