You have already had the thermostat changed, but the new one may be faulty. For the heating system to work, the thermostat must be completely closed until the engine is up to operating temperature. If it is open even a little, then the engine will never warm up at idle. I have seen many new thermostats that were faulty, so it would be best to have it removed and tested or another new one installed. Install a weather front on the van during cold weather.
Cold air flowing through the engine compartment cools the engine too much.
Block as much air as possible. A van is one of the most difficult vehicles to heat. It has a large interior volume and lots of glass area. Remember, a van will always take longer to heat up than a similar sized passenger car or truck.