The frost stat can be put into the hose reel input it will still detect the cold in the van and it will come on when needed,I put mine there in the last heater so it not only worked as a frost stat but also gave me a correct reading on the controller.
I have a main hook up on mine which I think is essential with these that’s why I paid to have them install it earlier in the year,when I charge I know it’s doing it’s job I can look at the quality of the charge in the back of the van on the charging unit they installed.
Varitech say if they turn on they are working the heaters that is but I found this not to be so with my old heater,I found if I had better battery life it produced hotter water with loads of trial and error.