I would guess your thermostat is not connected to turn on heat at the roof unit. honestly don't expect a lot of heat from a 1400 watt strip you're only going to get 5000 btu's out of it. The best money spent is for a heat pump, they put out about 14,000 btu's and still only using 15 amps of power (same amount of power as the heat strip).
Mark & Melissa
2015 Redwood RL38 , Kodiak disk brakes , 17.5" tires , 8K Morryde IS , 2020 GMC 3500 DRW