holding tank question

[Diehard]

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.

[chuckster57]

Do you have both waste tank valves closed? It’s common for seperate tanks to drain out one termination fitting, but there should be a dump valve on both that will prevent one from backing up into the other if they are working correctly. If your using the bathroom sink, it may be plumbed to the black tank.

[bobbecky]

When you run water into the shower, does the toilet water level rise? If it does not, then you have proven the sink drains into the black tank but the shower drains into the grey tank.

[Pull Toy]

Many campers dump the waste water into the nearest tank.

Using food coloring can help you sort out your plumbing without the need for wasting full tanks, or harming the environment.

Make sure you have empty tanks to start. Use two distinct colors. By testing each drain in question, you can get a better idea.

Good luck,