ive never heard of a caravan toilet drawing water from the drinking water source and cant really see how it could be hygienic if it where to back feed in any way which is probably pretty unlikely but if its possible then it will happen one day.
our thetford has electric flush and the fill tank is just above the housing for the waste tank, we no longer use the thetford pink fluid however as its been widely suggested that this fluid is the cause of the black algie coming out of the flush tank, we now use top and tail fluids such as aqua chem etc.