Dunno where it might be, but once you find it it's reasonably simple to get it sorted if it;s their newer screw-in style (likely).
Check the KUS/WEMA website to review how it works, but basically you just turn off power, disconnect the power wires, unscrew the sender, plop it into a bucket of cleaner (muriatic acid works pretty well), let it sit and bubble for a while... then screw it back in and remake the electrical connections.
Helps to do a flush/rinse/repeat a couple times beforehand, but the task isn't really all that onerous.
If it's not set up already for quick electrical disconnect/reconnect, add some spade connectors or whatever while you're at it, for next time. Which might be in about 3 years or more depending on how often you pump and rinse, tank treatments, etc.
-Chris