ALong the same line as 'Bookemdanno' says you may also pick up a 12VDC pump from the Boat section at WALMART and use that to pump water from your water jugs to fill the on-board fresh water tank...
My on-board water pump has a water hose line valve that either selects water coming from the Fresh water tank or from a Winterizing fluid jug to pump to the trailer water system. Using my setup would have to be re-engineered to pull water from water jugs to put back into the fresh water tank... My setup also has one-way valves in the water that would prevent water coming from the city water port to go directly into the the fresh water tank. I'm surprised you do not have a manual fresh water tank fill port...
typical water flow for trailers...
Showing my one way valves...
Some of the newer trailer have a water fill panel allowing you to fill your fresh water tank with different positions of the knobs..
Roy Ken