[Carl n Susan]

I do know the two tanks under the pass through compartment are the black and shower (grey) ones. The tanks lay perpindicular to length of the RV. The left handle (in the pic) is for the front tank and the right handle is for the rear tank. If you don't want to run water down the shower/bathroom sink to see which tank it comes out of, there might be an alternative way. Find the toilet down pipe which goes to the black tank. Based on where in is located to the two tanks should give you an answer.

As for the third tank, it is there believe me. It may be the handle has come off (it happens). Look for a hole in the frame or the coroplast around the tire area. If you really can't find it, then drop the coroplast along the driver side (easy, just remove some screws) so you can see the valves' location.