100's of models and mfg so it would be nice to know WHICH RV you have.....year/brand/model makes it easier for folks to help with info
Monitor panel is probably generic and with just 1 bathroom you most likely have just 1 black tank BUT 2 grey tanks
A galley for kitchen only and then a grey for bathroom sink/shower
Sink stopped up after you have cleaned 'P' trap and snaked line.......galley tank if FULL
Look around the roadside (drivers) trailer tires for the dump valve for that grey tank. It may tie into the main drain line OR it may need it's own hose hooked up to drain but it will have it's own drain valve