While camping in northern LP Michigan during firearm deer season, the temps never got above 22 for a week straight. I was using an electric space heater with the furnace thermostat set for about 55 - in theory, to keep the pipes from freezing. Well, the water lines froze up anyway. I turned the furnace up to about 80 to thaw them out, and I went through a 30# tank in 3 days. So for a rig the size of yours in cold weather, your LP consumption doesn't seem abnormal.
BTW, turning the heat up didn't thaw out the pipes, but (like chicken soup) it couldn't hurt. I headed south after three days of dry camping, hoping to get into warmer weather. Outside temps didn't get above freezing until I got into southern Missouri. The water lines finally thawed out - a week after they originally froze up. Fortunately, there was no damage from the freezing/thawing.