opinions on after market holding tank monitoring system

I installed the See Level tank monitors as part of our remodel.
Love 'em. Never regretted the decision. If you're planning on keeping your rig for a while, it's probably worth it.

I DO have one issue with them. When it's super wet outside - rainy, snowy, what have you - I can get erroneous readings on the grey and black tanks. They hang under our class B and are exposed to the elements. Once they dry out, they go back to working just fine.