I have a 2005 Grady White. I had not had to pump out the holding tank in quite some time (years) and then when i had to do it recently the cap would not unscrew. So i had to crack it loose with hammer and screw driver to get it off. Now I'm trying to find a new one but everyone that i've bought does not fit tight. Either size is wrong or threads are off. Anyone have suggestions where i would find the correct one?