Black tank reads full when empty  Usually toilet paper or waste film bridging the probe and tank wall — completes the circuit even with nothing there
Gray tank reads full when empty  Grease, soap scum, and food particles build up on probes over time — a slower, greasier version of the same problem
Fix for both  A commercial tank cleaner/enzyme treatment run through several fill-and-dump cycles resolves the majority of cases without physical repair
Prevention  Always dump with enough water in the tank to create flow, keep the black tank valve closed between dumps to let solids break down properly, and leave the gray valve open only when you don’t mind grease coating the probes on hookups

FIELD NOTE

A black tank left open at a campsite is one of the most common causes of chronic sensor problems. Without standing liquid, solids pile up dry on the tank floor and coat every probe on the way past. Keep it closed until you’re ready to dump with a full tank.

Motor runs, nothing comes out  Hair, wipes, or debris jammed against the impeller and motor shaft — open the housing, clear the blade, reassemble
Motor labors and cuts out mid-cycle  Almost always an undersized discharge hose — a 3/4″ hose over any real distance makes the motor pull excess current fighting backpressure and it thermal-trips; 1″–1.25″ hose is the fix for long runs
Won’t start after winter storage  Seals dry out and the impeller can seize — a penetrating lubricant in the discharge port followed by manually turning the shaft, then letting it sit, frees most stuck units
Intermittent operation  Check the pump’s inline fuse and wiring connections for corrosion before assuming the motor is failing

PRO TIP

Never run a macerator dry or without water flowing through the tank first — it’s designed to shred and pump solids suspended in liquid, not act as a grinder on its own. Running dry accelerates seal and impeller wear.

Dry P-trap  Sinks and showers unused for a while lose their water seal, letting tank gas rise straight up the drain — run water in every fixture periodically, even ones you don’t use often
Dry toilet valve seal  Same principle at the toilet — a dried-out blade seal lets black tank gas into the bathroom
Roof vent cap issues  A cracked or missing vent cap, or a vent that’s too close to a window or vent fan intake, can pull sewer gas back into the rig
Low water in the black tank  Not enough liquid to keep solids submerged concentrates odor — add water and a tank treatment after each dump

FIELD NOTE

Skip the “blue chemical” toilet-bowl products long-term if you can — enzyme and bacterial tank treatments break down solids and control odor without the harsh chemicals that some septic and dump-station facilities restrict.

How they work  Resistive heating pads bonded to the underside of the tank, typically 12V, drawing meaningful current — check the fuse and switch first if a pad isn’t warming
Thermostatic vs. manual switch  Thermostatic pads cycle automatically around freezing; manual-switch pads need to be turned on ahead of a cold snap and draw continuously while on
Battery draw in cold weather  Tank heaters are a real load on the house battery bank if boondocking in freezing temps — factor them into your power budget alongside the furnace fan
Heater pads don’t protect exposed plumbing  They protect the tank itself, not the dump valves, hoses, or exposed lines — those still need heat tape or tank/valve enclosures in real cold