Furnace using too much propane

It's been a few years since I've looked, but a RV furnace is the most in- efficient furnace there is. It has 2 fans, 1 motor. 1 pushes air threw the inside of the combustion. The other circulates air over the outside of the combustion chamber, that is heated to warm the RV. When the T-stat calls for heat the fan motor turns on for a pre set time, to purge the inside of the combustion chamber with air from outdoors, before the burner lights. When the RV T-stat reaches the set temperature the burner turns off and fan motor continues to run until cooling the combustion chamber to a preset temperature.

It seems to me that it wood be better to put a dampers on the combustion chamber to turn the air flow on/off.