[Carl n Susan]

Mt first reaction is yet another Thetford toilet failure. But filling the sink is unusual.

What sink was filled? The kitchen or the bathroom?

Have you verified the sink that filled does in fact drain to a separate tank? (Strange plumbing is not unheard of)

When you pulled the drain handles I assume they were all closed?

Did you notice which tank dominated the output or did pulling them all at once make it too difficult to tell?