I had the same problem,I have a 500 l purefreedom van mounted system,.
I put a post on here asking for help,one of the suggestions was to wrap top thread with Ptfe tape (lots of ) which is what I did I then placed a plastic bag over tank hole,due to lid is vented and will Let water pass,I then fitted lid no leaks !!
I was able to block lid vent as my systems has an overflow,which prevents tank imploding when pumping water,if you don't have an overflow you will need to either leave vent open in lid of find another way to allow air into tank