Thermostat operation is a bit more complicated than it appears. The Vanagon system makes its operation critical as it really is a mixing valve set up and in colder weather with all of that reserve cooling capacity the response time also becomes an issue. After OEM t-stats became no longer available I found that many just don't respond consistently. Hard enough even finding ones that are correct. The Meyle brand was disappointing. I have been using Whaler for a long time. On this last rip down to Florida the Whaler let me down. I just installed a Mahle TX 24 87D. First road test, WOW! Granted it is now ~50 here and sunny. Up to temp, gauge needle dead center in ~3 miles. I will be stocking these. And man the heat works again.
You can get them here.
https://www.amazon.com/gp/product/B00D3DH4J6/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Dennis