I had a hard time filling my fresh water using the fresh water fill. The water would always flow back out the fill hole. It turns out the fill hose between the filler neck and the tank was raised in the middle. This would cause water to flow back out the fill hole. My hose runs under my bed so I was able to move the middle if the hose down to fix the issue.
Not sure this is the case on yours because you are able to fill it 1/3 full before you gives you issues. Thought I would mention in it as something to check though. Good luck.
2015 Sunseeker 3170ds