We use our gas oven all the time. No added tiles. No problems. You might want to double-check the oven temperature is getting to where you set it - just with a simple oven thermometer.
Once the oven is hot, it doesn't take a lot of propane to keep it hot. The more often you open the oven to check the food, the more gas you'll use, but still... I would expect it to take a LONG time to burn through 1lb of propane in the oven.... like 6-10 hours?
That's a guess.... but the high burner on the stove top is a 9,000 BTU/hr burner, and 1lb of propane contains 21,000 BTU. The burner in the oven is long, but a lower flame... so probably close to the same heat input when the flame is on high... but, again, the flame goes off once the temperature is reached, and only comes back on to maintain temp. Our oven takes about 10 minutes to reach 400F (est. 0.1lb of propane used). We've often cooked multiple items in it, leaving it on all day long (i.e. at football tailgates).