What is your trailer model? Others with your exact model might be better able to help.
Are you certain there are not two inlets - one for a city water connection and one for the fresh water tank? These are sometimes together like in the picture above and sometimes spaced apart.
My trailer has two inlets for fresh water. One is the city water, which has a valve like you describe, and the other is the fresh water opening that has no valve. I use this to fill with a hose or with a funnel and a container if I need to.
The bleach mixture I use (1/2 cup bleach to every 15 gallons of FW capacity) goes in the second one.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD