Some trailers have a valve inside for diverting fresh water from the hose into the fresh water tank, if yours does, I can't help you out, but.... Many (most I've encountered with the exception of 5vers) have a "trap door" near the hose connection on the outside of the trailer. If you open the trap door, there is chute directly into the freshwater tank for filling purposes. Point your hose there if you don't have a filling attachment handy ( a smaller hose that attaches to your fresh water hose and fits into the chute).
If you do have an inside diverter valve, most I've worked with have been located next to the water pump, just above the inlet to the pump (between pump and water inlet connection).
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)