Tom, let me see if I can restate the symptoms:
1. You can cook with either tank open and you don't smell any gas around the tanks or regulator.
2. You cannot run the furnace with either tank open by itself, but only with both tanks open, in which case you smell gas at the regulator.
3. You have replaced the regulator 3 times for this same set of symptoms. The problem goes away for a short period and then starts up again.
I would guess that the regulator is not the problem.
If you're doing the replacement yourself, are you using LP tape on the fittings? I think regular pipe tape (for water fittings) breaks down and could start leaking after a short time.
Does the furnace fail to run on EITHER tank by itself? I don't know the Cyclone layout, but on our Landmark, the doorside tank has a regulator and a separate hose that runs across the rig. If that regulator starts to fail, or the hose doesn't pass enough LP, the furnace won't run from the doorside tank by itself.