UPDATE: I finally had my service appointment and their conclusion was that nothing is wrong with my rear heater.
They recommended starting off rear heat on "Hi" then backing off the temp as the cabin heats. They told me that it was normal when both front and rear were set the same (72 deg, fan 3 for example) that the front air feels hot, but the rear feels cool.
When using Hi temp setting for the rear, after a minute or so, I could feel warm air-- at about the same temp as the front with 68 deg set.
A quick check of running the rear heater in Hi, with fan=3, and outside temp 55, showed an initial consumption of 2430 watts, gradually ramping down over 7 minutes to 1215 watts and stabilizing. At the lower level, on a long trip, I would lose 3.8 miles of rated range every hour.
My conclusion: try to use just seat warmers and front heater if possible. If that isn't enough, the rear heater consumption is not a show stopper unless I am cutting it close on range remaining at destination.