I have a 2011 Copper Canyon (sprinter) and when I fill up black tank to the point I see water standing in the toilet and then run water into the sink I see the water rising in the toilet ???
Of course the belly is totally enclosed hiding the plumbing. I have the grey drain and black drain coming together past the dump valves and when I take storage cap off there is no sign that water was backing up the black drain past the valve.
I contacted Keystone factory, no schematics available.
Any ideas / suggestions would be appreciated.