You should read the section on gas safety before attempting any repairs.
All the information below pertaining to gas thermostats also pertain to the gas side of a Combination Thermostat.
The purpose of the gas thermostat is to control the temperature of the refrigerator when it is operating on propane. It does this by controlling the size of the gas flame. The capillary tube (or sensor) of the thermostat is a hollow, sealed tube filled with a gas that expands and contracts with changes in temperature. This movement of the gas inside the tube moves a bellows, which in turn controls the path of the propane through the thermostat.
The end of the capillary tube is positioned at a specific point on the evaporator (usually the fins) of the lower box. When replacing the thermostat be sure to position the capillary tube in the same manner as the old one. Failure to do so may cause inadequate contact, resulting in over cooling.
The thermostats below are drawn for easy understanding and are not mechanically accurate.
When cooling of the refrigerator is called for, the thermostat merely allows the full flow of propane at 11" water column through to the orifice and burner. See the picture on the left and compare it with the one on the right.
When the desired temperature is reached, the thermostat blocks the "open" pathway and routes the propane through the by-pass section. See above. On most thermostats, a by-pass screw with a very fine hole is installed through the by-pass pathway. Although some older thermostats do use an adjustment screw to control the by-pass flame, the screw mentioned above is not an adjustment screw. The reason for using a screw is that in can be replaced with a different sized screw in case the by-pass flame is not right. NOTE: the by-pass screw is changed entirely too often. If the by-pass screw was the right size for years, it doesn't suddenly become the wrong size. Usually, the only time a by-pass screw needs replacing is on a new installation--and then, rarely.
When the propane is forced through the small hole of the by-pass screw and then into a larger chamber, the output pressure is reduced to between 1 1/2" to 3" water column, depending on the model. Please note this range for low pressures is a range for all models combined; a specific model has a much narrower range for its own low pressure. The lower pressure, or by-pass pressure, coming from the thermostat causes a smaller burner flame designed not to generate cooling from the cooling unit, while still allowing the flame to burn. However, too small a by-pass flame will not heat the thermocouple enough to keep the safety valve open, and the flame will go out.
No or Poor Cooling
RV Mobile Inc. 11715 HWY 99, Everett, WA 98204
The owner of RV Mobile Inc. apparently suffered a heart attack and the original website was shut down.
It has been reposted here to preserve this wealth of information RV refridgerator information.