I have a 2005 Pace Arrow with a Thedford Tecma Silence Plus, and the only things that come to mind is the level sensor in the black water tank may be malfunctioning or perhaps the wrong type of toilet paper was used. This black water level sensor is supposed to stop the toilet from flushing when the level is too high in the black water tank so things won't overflow. Does this happen when you tanks are nearly full? Some toilet papers are too thick for the plastic impeller to chew up when flushing, so it's a good idea to use the RV paper, or test the product by putting a few sheets in a jar of water and shaking it after a couple of seconds. If the paper disintegrates then it's OK. If not use another paper.
I don't know if either of these conditions pertain to you, but I'll watch with interest for any other replies to this discussion, in case that it happens to me.
Oh I guess as a last resort, you could always call Thedford and see if they have a suggestion.
In any case, good luck!
2005 Pace Arrow 36D, UltraPower,
Blue-Ox TigerTrak Front & Back, Koni FSDs,
Safe-T-Plus, Magnum Wires, Scan Gauge II