Black & Grey tank readings

Either exterior sensors like seelevel, or clean the tanks from the inside. Grey tank you can dump in some Dawn or other detergent. I'll sometimes squirt some down the sink if I know I'll be dumping at my next destination. The drive gets it all stirred up and clean(ish). Black tank you can also install a flusher, or use a wand down the toilet if the black tank is right below the toilet.

I've had good luck with lots of water, and some Dawn in the tanks when I know I'm dumping at the end of the day's drive.