And why does the refrigerator need a heater? Is it part of the propane system or is it for defrost?
These are absorption cooling refrigerators. The heat is what supplies the energy to do the cooling. When you're running on 110VAC, the heat is supplied by the electric heater. When you're running on gas, the propane is supplying the heat. (In both cases, the electronic "brain" is powered by 12VDC from your battery; if you're on shore power, your converter will supply the 12VDC while charging your battery). Absorption cooling is explained here:
Note that some manufacturers also have an electric heating element around the doors to prevent frost buildup. Some of these models have a switch between the fridge and freezer doors to turn the heating element off and some have no way to turn it off. If you are boondocking (running strictly on your battery, with propane doing the cooling) this little door heater will kill your battery very quickly.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10