My 2016 317 RLOK takes about that long to fill too. I will travel with at least 1/3 tank for on the road use.
My overflow does the same as yours. I had the same problem again this weekend. I was asking other campers if they had the same problem, 2 guys said yes, apparently it's creating a siphon. They said they switch there valves to city water hookup to stop the siphon.
I wil try that next time we have to use the fresh water tank.
2016 Ram 3500 Big Horn 6.7 TD
2016 Jayco Eagle 317RLOK(rear living rm outside kitchen)