Welcome to the forums! Lots of friendly and knowledgeable folks here!
We have an AquaHot hydronic heater that we really like; but I suspect that would be overkill in a van installation. I assume the van has a diesel engine? If not, I would think dealing with two fuels (three if you have a propane stove) would be an issue. And if the engine is diesel, your mpg and ability to stay off the grid will be impacted. If you Google ‘diesel heater for van’ a lot of options pop up.
A couple of features with our Aquahot that I would look for:
It heats our water as well as our coach.
It draws its fuel from the coach’s tank, but its inlet stops about 1/4 from the bottom, so as to not let your heater leave you stranded.
Hopefully some members with more experience and knowledge will pop in soon.