sounds like it or the pump needs changing. also it could be a loose connection and it's not getting full current. for free pdf dowloads go to thetford website.
instruction below save you searching. if it still persists then contact thetford under 'ask thetford on the site listing.
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 (Worthing)
April 2011
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.