I run a 60 watt bulb in the water pump/holding tank area, and another on the other side at the WH, both connected to a timer....off in the daytime.
Then inside the RV I run a 500 watt heater along w/ an indicator lite hanging in the window, both connected to a thermostat "CUBE" 35-45 deg.
Never a problem in 15 deg weather
Rich
'01 31' Rexall Vision, Generac 5.5k, 1000 watt Honda, PD 9245 conv, 300 watts Solar, 150 watt inv, 2 Cos 6v batts, ammeters, led voltmeters all over the place, KD/sat, 2 Oly Cat heaters w/ ox, and towing a 2012 Liberty, Lowe bass boat, or a Kawi Mule.