Hmmm... if the grey and black tee into the same pipe where you attach your slinky after their individual gate valves then maybe if you left the cap on and pulled both valves they would equalize levels. However, that would make your life a misery when you finally had to open the cap to dump for real (probably not recommended to mix sewage into your grey tank either).
Just a (nasty) thought experiment.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam