During a little research spawned by a different thread, I found the attached diagram. I thought I would share it with the Forum!
Note the comments - " . . .not be installed where venting of water from the device during normal functioning will cause damage." Hmmm, stuck up inside a wall, inaccessible, too big to get through the small hole drilled, behind the basement wall, and above the Transfer Switch. Naw, if the valve vented water, there's no damage there . . .
FYI, I did find one place (an RV dealer in New Castle, IN) who Internet sells these valves (Vacuum Breaker Model 571), and I'm looking for others. It says it must be installed 6" (15cm) higher than anything connected to the waste tank. So, I would imagine that this little valve is slightly higher than the toilet lid inside that wall!
I'm thinking maybe looking for a spring loaded check valve to put inline. Hank, what do you think?
Ron