tank monitors

I do realize the tank monitors are not very accurate generally speaking. However, my experience has usually been the monitors show fuller than they actually are. I have a 4 tank system: fresh, black, gray, and galley. The gray and fresh work reasonably well but the black and galley read empty all the time. My gut feeling is the actual panel is bad. Considering the underbelly is enclosed, the actual panel is much easier to replace than the sensors. Has anyone else had this particular problem and found the panel to be the problem? Thanks.