My concern is that if I have too many batteries my truck's alternator will struggle to keep everything charged.
I have one AGM battery in my pop-up truck camper and an 18,000 btu furnace which draws down the battery in 24 hrs. So in the evening I turn off the furnace, run my Honda 2000 watt generator which powers a 900 watt electric heater and the camper's converter/batt. charger. I figure that if I add a second battery I might go longer between charges but it will then take longer to charge two batteries instead of one.