3500 drw 2011 coolant temp fluctuates rapidly. From 204-230 under heavy load. Increases going uphill
Replaced old thermostat still same issues.

Without a load I'm at 190-200

 

How full is your overflow tank?

 

It seems to be at minimum level constantly even when I refill, few days later it's down again

 

You either have a leak or antifreeze in your oil. At first I thought about your cooling fan turning on to reduce the temps going up a grade under a load, but the consuming of antifreeze makes me question where it is going. Take a sample of your oil and send it to a lab like Blackstone, they can tell you if the antifreeze is getting into your oil in minute quantities that are being consumed. It is relatively inexpensive around $25.00 or so. Just my 2 cents worth.