There can still be a buildup of soap scum, grease, etc that coats the walls of the gray tank. What type of sensors do you have, the kind that stick through a hole in the side or the square plastic stick type on that reads through the plastic side?
I had the same problem with the gray tank on my Itasca, which had the exterior sensors. When I cut a hole in the tank to install one of the Tornado rinse out systems, there was a 1/16" thick coat of gunk on the inside. It took several washings with Dawn and the Tornado system to finally get the sensors to start reading properly.
Dan & Ro with two Rescued Boston "Terrorists"
2015 Tiffin Allegro RED 38QRA
Previously '06 Itasca Sunrise 35A on a W22 Workhorse
'12 Jeep Wrangler Sport and/or '08 Honda Goldwing