Mine works in part with a flat plate heat exchanger. I also have a pump that can be used to supply hot water for quite a while w/o the Espar provided the engine is hot. The disadvantage is you need to put the lines and flat plate where it won't freeze or you can rupture the flat plate and contaminate the potable water.
I'm not to sure about the D-5 or newer units but was told they work more like a heater core. The Airtronic uses less power than the Hydronic. But both of them pull more power when they first turn on and taper off. The Hydronic cycles more as it senses that the water circulating in the system needs to be heated. It runs full power at first because it's heating the engine coolant and that takes time. But you get hot water almost instantly (a couple of minutes) at the tap. It just takes several minutes before the block water is warm enough that the unit cycles down. If you're not worried about heating the engine you just turn it off. There is no hot water holding tank. If you want hot water again just turn it back on. The unit itself is fairly small:
The Airtronic uses more power if it's set to produce higher temps just like a gas heater in a house. Mine is very quiet and only pulls an amp or so when set to low. Open the door to go outside and cold air can cause it to ramp up, make more noise, and pull a little more power. On super cold days it might run louder and pull more amps to keep the cab warm. Without a top I doubt you'd need to run it on the high setting provided the van is insulated well.
Again I know nothing about the other units... how loud they are or how much power they pull.
Espar products are expensive and there are several lower priced ways to go to heat water or the inside of the van, especially one that is well insulated and has no top.