What is the heater cycle
If your talking continous run all night, your going to wake up cold one morning
You talk like it's coming on and running for hours on end
Your mentioning battery voltage under load at one hour mark
Technically, with new batteries 7 amps *8 hours = 56 amp hours approx 30% of capacity, leaving you with plenty of battery
But that is good batteries with no sulphation
We can't really answer your question
You still didn't answer, where are you measuring voltage
You want us to give you a specific number, not to go below, when to turn off heat and bundle up , or start charging
We can't do that