On the thetford site are downloads of drawings
and it's listed as 19 not 13 is listed as pour spout.
I think if you have a c/van camping shop nearby or a service centre take the tank in and they may do the job for a few quid as it's such a small job. It's better than trying and breaking the tank they aren't cheap.
Have a look at the site www.thetford.com select parts
select permanent toilets select the first in the centre c2 [it covers 2 3 & 4 scroll down to drawing follow on for parts list. to copy Scrn print [takes two hits for all drawing ]paste to word file. Scrn print the parts list do the same save each by copying in to photoshop 'new' then save each as a jpg. you can the zoom in for better view.
Bob
September 2008