Tim, it appears that CATCON is no longer in business. Will not answer phone calls or e-mails. Thanks for the info anyway.

Yes, my waste tanks are mounted on a slant.  They sit on wedges of Styrofoam, with the passenger side elevated for good drainage.

I mounted the SeeLevel sensor strips on the passenger side of the black and gray tanks.  Since it higher than the driver's side, they don't see the first few gallons of material, and except when the tank is exactly half full they are off by a bit.  

I carefully added a gallon of water at a time to the gray water tank and recorded the level indicated on the SeeLevel monitor.  It is off by 2% through most of the range, but 5% in the first and last 10% of the range.

I'm happy with that.  Of course, the fresh water tank is not mounted on a slope, so the SeeLevel sensor gives very accurate reading there.

My tanks are not mounted on a slant, but have slanted sides. Viewed from the side, they are a trapezoid, with the across-the-top measurement of the tank about 3-4 inches more than the across-the-bottom measurement, making the sides go down on a slant like a pyramid in reverse.

That would certainly affect the SeeLevel readings, but I would still install it anyway.

When I added a gallon at a time to our gray tank and recorded the SeeLevel readings, I created a spreadsheet of the results. I printed out the results and taped it to the wall above the display.  At a glance, I can tell that what the actual level is for each reading of the displayed values.  

The system displays in increments of three or four percent.