When you say '... filled it again ...' do you mean all the water drained out of the tank?
Each fresh water tank has a valve to drain the tank.
Also, each fresh water tank will have an overflow tube that is also a vent tube. Water flows out of it when the tank holds all the water it can. It also lets air out when you fill the tank and lets air in when you draw water from the tank.
You should be able to see the water flowing out of the tube(s) when the tank is full, but it will not drain all the water out of the tank.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.