Quote from www.horstmiracleprobes.com

"Most RV-ers agree that the weakest system of any RV is the ability to monitor the level of waste water in the gray and black holding tanks. The failure rate of existing measurement systems is so high that many RV-ers simply guess when they have to empty their tanks. Consquently, RV-ers dump more frequently than necessary. Certainly not the most enjoyable part of RV-ing. "

I havent tried these yet but I've read some good things about them. Has anyone here used them?

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2001 Hi-Lo 22TL
2010 F150

Ordered these probes today, as my gray water tank reads incorrectly on my 2808C. I've flushed this tank, used a thetford product to clean the probes, flushed the tank several additions times, all to no avail. Oddly, my black tank has not had the same problem. I'll post my thoughts after using the probes for a period of time.

I installed a SeeLevel Tank Monitor, by Garnet. The monitor actually displays percentage of the tank levels (ie. 35%) You use existing wiring and place adhesive monitoring stripes on the side of the tanks. They work great and the only time I had any problem was when the humidity was real high. I understand you can coat the strips and eliminate that problem. The fresh water tank is insulated but I cut an access hole with a dremmel tool then pop rivited it back in place.
SeeLeveL Gauges by Garnet Instruments - About Us

I just got back from my fifth camping trip after installing the Horst Miracle Probes and they have performed flawlessly. I've never had this many true readings in a row from the time I bought my unit new in 2001. As far as I'm concerned, the rv manufacturers should install the Horst sensors instead of the standard "error reading" sensors. It's money well spent.