[C320-list] Waste Tank Question

[Martin Rosenberg]

How do you check the status of your waste tanks? The PO of my boat stored
it for the winter with the tank FULL. I didn't know and since I wasn't
using it assumed that it was empty. I finally removed the vent hose and
unscrewed the fitting to inspect and found that it was inches from the
vent!!
Anyway, short of emptying it every week from paranoia, I need a reliable
way to check the tanks. I've been looking at different tank monitors and so
far, the iSeries from Tech-Edge is the only one that looks decent. Since
it's over $200 I'd like to get some references. What do you all use?
Martin Rosenberg
"Avalon"  #823