I have an 04 HR Imperial with AquaHot. There are 5 heat exchangers ( small radiators with small fans) in the coach. There are three in the living area, 2 next to the dining table and one under the center console between the driver and passenger. In the living area there are also 3 unused grill covered outlets that look like heat vents but have no function. There is one heat exchanger in the toidy room. There is one exchanger in the bedroom under the sink at floor level. There are also 2 more grill covered outlets that have nothing behind them but the open space under the cabinets. These actually serve a function, that of providing return air to the exchanger under the sink.
If you have AquaHot and are looking into grills that seem to have nothing behind them, that's normal!!
Ron