on the inside level with the water inlet you may find a pressure control switch under the seat in the cold inlet pipe[blue pipe] it's white has a knob at the top.
close this switch turning the knob clockwise.
open the cold tap on the kitchen sink turn the knob back anti clockwise to open the valve until it starts to click, then stop turning it. turn off the tap the pump may run on for a few seconds then turn off. i have the same problem and have to carry out this adjustment every time the pump runs on. it sometime does this when the water in the barrel reaches close to empty. . so you know what to look for type in google search engine whale leasure pressure switch various will be shown yours may be the white one.hope this helps. if you ever have problems with the thetford toilet i'm adding the access to thetford details, hence this reply being so long.
uk & europe www.thetford-europe.com
australia www.thetford.com.au (even get request from australian campers)
australia site has stockist list.
home page select english {if in australia forget this selection it doesn't come up)
select customer service (top line)
select products from top line
select bulit in or portable which ever applies.
select user information from list on right.
select repair instructions from list on right
select built in or portable which ever applies.
select select your toilet series model number from right list.
you will now be in the pdf listings for your range of toilets just select those that apply to your toilet and down load them they're free.
as you go through watch for 'ask thetford' on left listings
you may not find what you want and you can send them a message.
for safety reasons which i agree with thetford do not offer repair advice for fridges. refer to service centre
Bob
March 2011