First, select the electrical function and lightly and quickly touch the tube the electric heating element is housed in to see if it is getting hot. If not, the element may be the trouble.
Second, select the gas function and see if the burner actually turns on. If not, that can be the problem. either no gas or a problem with the gas system or a defective burner part. You should hear a clicking sound until the burner lights as the system trys to initiate the ignition.
Third, remove the outer cover for the refrigerator and look att he bottom of the heat coils. You may have to remove a sheet metal heat shield around the propane burner. If there is a yellowish residue then you have a leak in your cooling system and it requires replacing the entire cooling system (it comes as a complete unit) or a new refrigerator.
The entire system consists of the box and the cooling unit. Replacing the cooling unit is MUCH cheaper (about 1/5th the cost) but you have to be a bit mechanically inclined.
The parts and instructions are available at :
RV Cooling Unit Warehouse
I have changed one twice, first with a rebuilt unit and again with a new unit when the rebuilt unit did not live up to my expectations. They were SUPER about it and gave me full credit for the unit I returned. There is also full detailed instructions downloadable on their website. When I was finished the (old) refrigerator worked better than it ever did even when brand new.
If you decide to do this feel free to contact me as I feel very comfortable helping you through the process.