Found this in a search for winterizing Tankless WH.
It probably would apply to your inquiry…
https://www.thorforums.com/forums/sh...&postcount=103
Winterizing your RV Tankless
“I ran this by Thor for winterizing my 2018 Vegas 24.1, with tankless hot water system, and got approval.
First, I plan on blowing out the system.
Then I will fill my fresh water tank with RV antifreeze, since I never use the tank. Next, I will pump the antifreeze through out the system in both the cold lines and hot, passing through the water heater. Lastly, I will put some in both the gray and black tanks.”
Although further down in the thread, a post reminded him to completely empty tank, that he only needs abt 2 gal of anti-freeze/water mix in his entire system.