The spec is for maximum load, so burner on with lights and motors if any.
Coil for gas control draws some, control board not so much.
So if not running very small load.
Set it to real cold while plugged in, place water bottles to store heat, then set for normal when arriving.
Battery capacity is easy, lookup the manufacturer data, starting batteries are listed with cranking amps but the amp hours could be determine by contacting manufacturer if not found.
Bear in mind that us when fully charged and in good working order.
Also, rating is usually stated as the eight hour rate, say 80 amp hour Battery, this will support a 10 amp load for 8 hours, it will NOT support a 20 amp load for 4 hours
The higher the load the less you get.
The opposite is also somewhat true, a lesser load will allow more capacity than indicated.
What does all of this mean?
Your unit will draw from little amps to more amps creating an average load somewhere in the middle, you use the maximum to determine the minimum run time knowing the average will be less.
Confirm the Battery rating, the manufacturer may have a rate sheet listed by load
Take the amp hour rating at the load closest and higher than your load(should measure to be sure) , divide the capacity (in amp hours ) by load (in amps ) to get the run time in hours.
Only works if Battery is GOOD, FULLY CHARGED and the load is known.
Tony & Lori
1989 Country Coach Savannah SE