Try the ice method. Throw a bag of ice down the toilet after you dump. Theory is it'll clean the sensors while you drive. It's not perfect by any means, but it helps and we have used it with pretty good success.
As gross as it sounds, just looking down the toilet throughout your trip is really the best way to know. Trust me, you will know when it's time.
Six of us have camped for a week without having to dump. That's the longest we have gone. That was two adults, three teens and a little guy.
We conserved water like when we were tent camping and encouraged the kids to use the campground toilets if they were out and about (but didn't make it a requirement).
Happy camping!