Once I get my dash put back in place after rebuilding the blender box,
I did mine a few months ago using the cut window approach from underneath. I figured that my well cared-for van would not need a heater core for some more years. Two months later, it went out. Now after all that contortion to avoid heavy disassembly I've got to take it all apart anyway.
So, do your heater core now! You won't regret it. Rockauto has best price I found.
For people that camp in the boonies a lot, have a "real" fridge and use solar, I highly recommend Battleborn batteries (or other quality LIFEPO4) for a house battery upgrade. Not just the increased capacity/weight-size ratio, but the quick charging profile, etc, etc., tremendously increased life in storage as well as life cycles in use. A good lithium battery should last ten years. Of course the value of a high performance house battery depends on your usage as well as budget for initial outlay. But worth considering before spending another $ on the old finicky lead acid tech. The Eurovan is a perfect candidate with the under seat location. Lithiums should be inside the vehicle, not outside it.
cheers