Found this in a search for winterizing Tankless WH.
It probably would apply to your inquiry…
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.