The glow plug ,fan and pump on a night heater draws about 4 or 5 amps so if you leave it running for 10 hour thats about 40/50 amp/hour draw
Lowest rated battery even on a 7.5 tonner is about 600 amp /hour and you got 2 of them so it shouldnt even touch the rate of charge of the system even if it was in burn mode all night.