You are right, early morning and late evening are the most vocal times of the day. I've never seen turkeys that didn't gobble first thing in the morning, before flying down. Once they are on the ground, I've seen them get quiet pretty quickly, and some mornings and some evenings they aren't AS vocal, but I've never seen a time that they didn't at least gobble some while on the roost.
Once you've got the roost located, it's then a matter of being there early enough to watch and listen for the direction that they move off, and go from there. ALSO, get close to the roost area early afternoon, and watch and listen to the direction that they come from coming back to the roost, and learn from that.
Otherwise, walking the area mid morning to afternoon and doing some calling, you may be able to strike them up and learn where they ahng out.