I have a Bailey Pegasus 514 with a Thetford toilet, electric flush.
On the top of the cistern, near where you press to flush the toilet is a light that should come on only to warn you the toilet is nearly full and needs emptying.
We just come back form two weeks in Cornwall. On the second day the warning light came on and no matter what I did, other than turn off the power, would not go off. Even when the waste tank was not in the caravan.