I had the same issue, found the larger fill tube is hard plastic and does not seal to the filler port very well. Removed the white sidewall filler and really cranked down on the clamp till it had a good bite which I could not do through the kitchen cabinet. If I remember correctly, the center of the filler port, where you insert the hose to fill the tank, unscrews from the main body. Caused some problems reseating it in the whole assembly, you may find it easier/better to remove the whole assembly.
Kind of hard to explain, hope this is clear enough. By the way, I think this is how water got into/under my floor, eventually rotting out the lower plywood.