Gray and fresh tank monitor probes don't foul as easily as black tank probes. There's no TP or poo to get hung up on the probes. We've never had an issue with gray or fresh monitor systems, just the black tank and that's the only one with the "need to listen for a sound change".....
But, I can say that we also know from experience, how many days we can camp, how many showers we can take and how long the fresh water will last without needing to monitor the status. After using the trailer for a while (4 or 5 camping trips), the monitor system just doesn't get used, we already know the tank status before we walk by and push the button.
It's sort of like knowing how many hot dogs we eat for lunch, so if we're going to have hot dogs for 3 lunches, we know how many to put in the refrigerator and how many packages of buns to bring along... For us, the tank status is pretty much the same, we know before we leave home how full the tanks will be when we break camp to leave the campground to head back home based on past camping trips. The monitor system just doesn't get used much, if any at all.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS