I have a mobile home with a gas fired downflow furnace. The furnace was installed in 1995 and is manufacturered by Coleman. In order to save on the energy bill I am considering lowering the setpoint on the T-stat below the 55 deg F minimum position . I can accomplish this by using an old mercury bulb type thermostat that I have from the previous furnace system.

Will operating the furnace to maintain lower space temperatures damage or shorten the life of the furnace?

Lower temperature = less usage. I cannot see this factor causing a problem BUT if your unit is a high efficiency condensing model then I would take care with how much you drop the temperature to due to the condensate freeze up in the drain line if it is drained under the trailer.