Bob,
In my opinion, the tank sensors in your coach are very reliable if they are keep clean. For example, yours are 16 years old and they apparently still work. The system works by measuring the conductivity between the two brass probes that are inserted into the tank at different levels. If the probes get dirty with wet toilet paper hanging across the probes or a layer or grease bridging the gap, they don't work.
I have been fulltiming in a 2000 Marquis for nine years with the same design tank monitors, and they still work well if I flush the black tank on a regular schedule and clean the gray tank occasionally. I clean the gray tank by filling it about 1/2 full of water and pouring in about 1/3 of a bottle of cheap laundry detergent, that I buy at the dollar store, before a long days drive.
The advantage to the SeeLevel is that the probes do not get dirty. However, in my opinion, the tanks should be cleaned periodically anyway, so the extra cleaning to keep the sensors working in no big deal. It is certainly less of a job than installing the SeeLevel system. But if money is not an object, you can have someone install it, and it will make life simpler.
Gerald
