the only silly question is the one you do not ask
first thing to remember is any anti-freeze can be poison.i contacted thetford for the idea of using an anti-freeze in the toilet flush header tank,this was there reply by email.
_______________________________________________________________
Thank you for your e-mail.
Any antifreeze used should be non acidic and non corrosive. Generally any non toxic anti freeze that is mono propylene glycol based should be ok. If using anti freeze please adhere to the manufacturers instructions at all times.
Please refer to your Thetford Users Manual with regards to using the toilet in the winter. Please be aware that Thetford accept no responsibility for damage caused by any anti freeze products.
Kind regards,
Thetford Customer Care.