We, too, have had good luck with the simple, wireless, two sender AcuRite unit, ordered on line, that--while taking batteries, they haven't been replaced in more than a year. We didn't have to worry about access wires for transmitters, can place them anywhere in the freezer and refrigerator spaces, and also keeps a record of high/low temps, just in case you're interested.
With varying conditions (particularly on non-residential type frig units) we feel more confident in the performance by monitoring actual temperatures.
Gotta keep the important stuff cold!
Side note; we take them out of the fridge and leave them on in the fifth. We then have an idea how the temps have ranged while it's in storage.
(AcuRite 00986 Refrigerator/Freezer Wireless Digital Thermometer)
Uusally on sale somewhere; shop around.