The above reference as to where to find the tank sensors is correct, but if none of your readings work, (black, gray, LP) I'd start at the indicator panel and see if you have power delivered to the panel. If you do, then start tracing grounds. Failure of a single sensor will not kill the whole system. I've lost a single sensor and everything else still worked until I got my sensor working.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad