Lots of talk on knowing how full a tank is, I don't see this as a need in a multi tank setup. We always start with two full tanks, when the active tank goes empty, flip the active tank lever, and refill, use the other tank until empty, and repeat the process.
I don't need or care how full the active tanks is as I wait until empty to refill. I only refill a partial tank once home from a trip. Then I flip the selection lever to the spare (full) tank, and take the partial to be filled so we have two full tanks at the start of the next trip.