What type of thermostat?
Suspect it's something like this (I found it on another site, better than I could explain, so I'll use their words.)
Are you switching the switch on the thermostat ON to heat and raising the temp to make the heater come on OR are you switching the other switch, on the thermostat, to FAN ?
The fan setting on the thermostat is for the air conditioner fan and will not work on battery.
The heater had its own seperate fan motor inside it. Switch to HEAT on the thermostat, turn the temp up above the room temp and wait for the heat to come on.
If you have a Dometic Control for the Furnace/AC, here is another piece of info to tuck away.
When you are plugged into shore power and want to run the furnace Without having the AC/Heat Pump Fan come on you need to set the Fan to Auto on the control. Otherwise when the furnace cuts in the AC fan will come on also and blow cool air.
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW