The sensor should be near the streetside bottom of the black tank, probably under/behind your water bay. Beaver Coach replaced ours a few years ago. The devices get clogged from the rig sitting unused for months, and the tank not being flushed well first. Gunk dries inside the sensor port and that device is a goner. I use my factory Sani-flush with almost every dump, and don’t leave my valve open in campsites, emptying only all at once when leaving or when the tanks are near full.
That said, the last time we were out, 3+ years ago, the sensor reading was acting up. I emptied and flushed several times until the effluent ran clear, and then added down the toilet some Calgon Liquid Water Softener, some Tide detergent and or Dawn, and as I recall a couple bags of ice with a couple gallons of water, and made the drive between campgrounds. Dump and flush, and it worked fine again. It’s been okay since, even while parked here at the house, only used a few times by visiting relatives (of course followed by another dump and thorough flush.
Some report success removing and cleaning the device, but that may not be successful. Just empty the tank (obviously), remove the RJ11 “phone” connector, and unscrew the Catcon sensor. It works by sensing the weight of the fluid pressing on it, which is why it requires calibration for each tank size. A tiny amount of dried gunk in some micro-spot can foul its abilities.
A new Catcon isn’t cheap for what it is:
https://shop.catconproducts.com/product.sc?productId=15&categoryId=2 or
http://www.nwrvsupply.com/139/16622728.htmlSome have happily switched to external sense technology, such as SeeLevel brand, but if you’re not into changing things too much, just switch out the Catcon and try to keep stuff from ever drying on it.
As to the smell, our shower trap gets sucked dry when the gray tank is emptied. It shouldn’t but it does. I just add a cup or two of water to the drain and the tank odor is gone until the next dump, and doesn’t always reappear then. If you’ve checked or changed your undersink vent valves, the smell could be from a roof fan allowing roof vent fumes in, a dry washing machine trap, or residual effluent gunk under your floors from the pipe leak you mentioned. Our Flex drain hose cracked and leaked kitchen sink water unseen under the stove, across under the floor tiles, and down into the Dirt Devil vacuum unit on the opposite side of the Coach. The clue was the loose tiles, and of course the smelly, wet vacuum bags I eventually followed my nose to.
Joel