One way to tell is to run water in the tub,sink till you see the levels rise do this after you know they are both empty. There is products to put in the tanks to clean them but they get tarnished and dirty and sometimes won't come clean they are other people here in the forum that have cleaning ideas too you can try. The slow drain tank could be the plug from when they cut the hole in the tank and did not pull it out, if it's a cable system could need adjustment.
Mark Anderson - Nebo NC - western NC - RV Restoration tech - 9 Doggies - Outdoors person
1990 33ft Wilderness Cimarron 33X