Got it guys, I played with it this afternoon after work
City fill the water pressure is enough to open check valve I guess
Country fill would be from gravity feed like water can is not enough pressure to open check valve so by reversing the filter screen it puts pressure on the ck valve and you can pour water in.
Thanks for the input
Also drained my hot water tank it had a high sulfur smell this weekend wasn't to good in the shower stunk up the camper