If you don't see any leaks on the ground , those van has an issue with a leaking heater core.inside the dash, the one that creates the heat
I would do what i mentioned above and retest , drain, coolant flush, reverse flush and bleed system .
if you still losing coolant ,I would look into the heater core
Please let me know if you need anything else on it, just reply back and we will continue
A 5 Star Rating is much Appreciated
Please keep in mind that when you rate me , it does not end our conversation.
you can always come back to this conversation and ask me any follow up question you may have on this issue
(you can copy this link page & paste it in your browser when ready and it will open up our conversation again)
ASE Certified Technician