Gray Water not empty, only 1 gate valve

Yes, this is the manual I have. Black water not overflowing into the shower drain when full. Must have a flapper valve in-between to separate the 2 tanks. When I get home I will flush and empty the Black water tank, open the cap and valve. Fresh water into sink should flow out w/o backing up???

Dang a combined black and grey tank?? You should be able to confirm that next time the shower water overflows the drain.

I don't have access to the tank to sound, buried under the floor, Since Six-Pac is no longer in business I would appreciate input from anyone with a similar experience. Thank you

this manual covers your model year. It shows only one combined holding tank for black and grey

http://www.six-pac.com/Newer-Six-Pac-Owners-Manual.pdf

Thanks for the recommendation, information about the grey water waste is not in my owners manual or the single valve operation procedure. Just states 2 - 38 gal holding tanks. The electronic g/w tank indicator may be faulty but, the tank does not seem to have the 38 gal capacity as indicated. (floor drain in shower overflows)

Since there are no codes for TC construction - the builder can make any kind of design that is continent for him, not necessary for user.
The single valve operating 2 tanks is unheard about, but good idea is to get camper manual.
The holding tanks level sensors are always lousy as dirt can stick to them, but do you have access to see or tap the tank? That should give you some indication.

Don't have a truck camper but I'll bet one valve serves both tanks. Can you hook up the dump hose, open the valve and then turn on the sink? Then either listen for water running in the hose. Or pick up the middle of the hose and see if the hose starts filling with water.