Welcome to the forums
Its extremely uncommon to have two fresh water tanks. Are you certain that indeed you do? The tank fittings should be readily available if indeed it is a faulty fitting, the biggest issue might be getting access to replace them. Do you have insulation or plastic covering on the bottom of the trailer covering the tanks. By removing part or all of it in the vicinity of the tank and putting in a few gallons you should be able to find exactly where the leak is without more water ending up soaking insulation etc. Hopefully someone with a similar model or water setup comes along to help troubleshoot.
Daryll