get one of these or similar type...
digital thermometer with remote sensorI put one in my slide in camper fridge...keep the readout in the cab. I know what the temp of my fridge is going down the road, or sitting in the house waiting for it to pre-cool.
Something to have when you go camping...You don't have to keep opening the door to see if the fridge is cold, letting cold air out and warm air in in the process. Also, it lets you know if there is a problem before it really becomes a problem. Nothing is worse than a fridge full of bad food because you didn't know it quit.