I am having some difficulty in visualizing your setup.
Sometimes water tanks are "slaved" together.
I had a boat that had two water tanks, and they were simply connected in a WYE configuration. The tank monitor was only on one tank as the water levels of the two tanks (eventually) leveled so that the tank not being read by the monitor had the same amount of water in it.
While the boat didn't have a drain, If both tanks are WYE'ed together, it would make sense that only one drain would be needed as it would still drain both tanks.
Perhaps your tank configuration is similar, either connected in a WYE configuration (both tanks parallel), or if one tank is physically higher than the other, perhaps it feeds into the lower tank. If the lower tank has the monitor, then it would not measure any use until the upper tank were drained.
Also, on my boat, there was only one fill tube as it filled both tanks simultaneously.
I'm guessin' you have a similar setup.
The only thing that works on a RV is the owner...