Holding Tank Dilemma Update:
Ok, so this is pretty embarrassing but I thought I'd provide feedback in case anyone else is suffering the same problem. It turns out that the tank that we were sure was empty was actually the BLACK WATER tank. Not the grey water - which was in fact full despite what the obviously malfunctioning monitor said. What is especially troubling to me is that my hubby just ASSUMED the valve he was opening (into a bucket, no less) was the grey water valve. I mean, REALLY!!, why would anyone make that assumption! Of course, I'm guilty too because I just 'assumed' the valve covers were marked. Very lucky for him and the rest of us that the BW tank was actually empty. (we live on a farm where there is a lot of animal fecal matter around anyway - but still, EEEWWWW!!).
So, the lesson to me and possibly to others is 1.) don't trust your tank monitors; 2.) Don't just assume which valve is black and which is grey. 3.) Don't always assume your spouse actually knows what they're doing. :-)
Anyway, it's all good now. Thanks to you all for your helpful advice.
Sue