You can always look down your toilet with the flush valve open and a flashlight to see "actually and absolutely" how full your black tank is.
The grey tank will back-up into your shower floor when it's full. You can then easily off-load some grey water from it into your black tank to get by for awhile until you can empty your grey tank.
However, I keep our sensors accurate through regular use of this in the grey/black tanks after each emptying of them:
http://www.amazon.com/Happy-Campers-Organic-Holding-Treatment/dp/B007S0H5DU/ref=sr_1_2/182-8018942-7067203?ie=UTF8&qid=1381300690&sr=8-2&keywords=the+happy+camperI've experimented with many tank additives to keep the sensors accurate, but this one above has worked out to be the most reliable on keeping the sensors working correctly.