thetford c3 cassette filling while driving?

the electric none return valve is my guess either flush button not completely disengaging or the electric none return valve is not closing as most motorhomes the toilet flush draws water from the main freshwater tank.
try thetford they have free repair pdfs but you can send them a message in 'ask thetford'.

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
May 2011