We put the Airtronic D4 in our pop top and love it.
Considered the Hydronic D5 but passed because of complexity, noise!, electrical demands! and damage to a flat plate when in freezing weather. Cost was a consideration, but would not have been a deal killer, if function had prevailed. In an insulated (windows covered with some insulation) fixed top rig, the D2 maybe sufficient and for a couple of hundred more than the D4, you could get two D2's.
Ducting for the D4 is 3" out of the unit but can be flattened to an oval if space is that constrained. Two outlets or even one central out let with deflection capability is all that is needed. I was pleasantly surprised how well the heat distributed from one central vent on the floor even with the top up and zero F outside.
There are other solutions for hot water if all you want it for is hot showers.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com