Intriguing!
Hydronic does look interesting, and to have some real functional strengths!
Still not sure how they would save much space in the interior though, as they appear to potentially have a lot of separate components spread through the van or RV?
To that point....for someone doing a build-out from scratch, with a totally-empty/gutted van, they seem like they are a solid type to consider. Running the extensive plumbing seems like the biggest bit of work/planning, no? Diagrams of hydronic systems seem to show lengthy networks of hot water lines running throughout the van's interior (and in some cases into the engine compartment) to connect to a network of radiators.
Would retrofitting a hydronic system into an already-built-out interior be a big deal? If it requires tearing out half the cabinets to run water lines everywhere, and to also install the actual heat-emitting radiator units here and there, it seems for that kind of consideration it might not be as attractive an option as a self-contained heater unit.
Like everyone here, I'm just looking to learn.