One thing you could try is to fill the fresh water tank, then try filling the water heater using the water pump. If it doesn't leak you could still go on your boondock trip and have some knowledge that the leak is associated with the city water fill port.
Another (and perhaps better) idea is to call a mobil RV Tech person to come to your location and fix the leak.
Also, consider putting the type of TT you own in your signature so others with that model can offer more specific advice.
Welcome to the forums!
2014 Micro Lite 19fd
2015 F-150 5.0L V8 XLT Crew Cab, 4x4, Tow Package, 36 gal tank, 3.55 locker, 1891 payload, Integrated Brake Controller, Roadmaster Active Suspension
Wooden Spoon Survivor