I have one of these:
You can order them from Bed, Bath & Beyond.
Great during camping, and it will read out in the truck going down the road, too. If you are leery of running the propane, you can at least monitor the temps between campsites.
With our Roo 19, we could go about 4 hrs before the fridge got too warm for the DW's preference. Our Mini lite's fridge seems bigger, so we've gone to running the propane,
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10